Development of the baby in the uterus in mammals. Embryonic development of mammals. How does a butterfly develop?
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The characteristics of the development of mammals will cover issues related to the structure of germ cells, fertilization, features of cleavage, gastrula formation, differentiation of germ layers and axial organs, development, structure and function of the fetal membranes (provisional, or temporary, organs). The subtype of mammals is very diverse in the nature of embryogenesis. The increasing complexity of the structure of mammals, and therefore embryogenesis, necessitates the accumulation more
nutrients in the eggs. At a certain stage of development, this supply of nutrients cannot satisfy the needs of a qualitatively changed embryo, and therefore, in the process of evolution, mammals developed intrauterine development and in most animals of this subtype a secondary loss of yolk is observed by the eggs. Sex cells. Fertilization. Splitting up
. The most primitive mammals are oviparous (platypus, echidna). They have telolecithal eggs, meroblastic cleavage, so their embryogenesis is similar to the development of birds.
In marsupial mammals, the eggs contain a small amount of yolk, but the embryo is born underdeveloped and its further development takes place in the mother's pouch, where a connection is established between the mother's nipple and the baby's esophagus.
Higher mammals are characterized by intrauterine development and nutrition of the embryo at the expense of the mother's body, which is reflected in embryogenesis. The eggs have almost completely lost their yolk for the second time; they are considered secondary oligolecithal, isolecithal. They develop in the follicles (folliculus - sac, vesicle) of the ovary. After ovulation (rupture of the follicle wall and release of the egg from the ovary), they enter the oviduct.
Mammalian eggs are microscopic in size. Their diameter is 100 - 200 microns. They are covered with two shells - primary and secondary. The first is the plasmalemma of the cell. The second shell is follicular cells (see Fig. 37). The wall of the follicle is built from them, where the eggs are located in the ovary.
Cleavage in higher mammals is complete, asynchronous: an embryo is formed, consisting of 3, 5, 7, etc. blastomeres. The latter usually lie in the form of a bunch of cells. This stage is called morula (Fig. 62). Two types of cells are distinguishable in it: small - light and large - dark. Light cells have the greatest mitotic activity. Dividing intensively, they are located on the surface of the morula in the form of an outer layer of trophoblast (trophe - nutrition, blastos - sprout). Dark blastomeres divide more slowly, so they are larger than light blastomeres and are located inside the embryo. The dark cells form the embryoblast.
The trophoblast performs a trophic function. It provides the embryo with nutritional material, since with its participation the connection between the embryo and the wall of the uterus is established. The embryoblast is the source of development of the body of the embryo and some of its extraembryonic organs.
If several babies are born to animals, then several eggs enter the oviduct at once.
Splitting, the embryo moves along the oviduct towards the uterus (Fig. 63, 64). The trophoblast absorbs the secretion of the glands. It accumulates between the embryoblast and trophoblast. The embryo greatly increases in size and turns into a blastoderm vesicle, or blastocyst (Fig. 65). The wall of the blastocyst is the trophoblast, and the embryoblast looks like a bunch of cells and is called the germinal nodule.
Rice. 62. Scheme of crushing a mammal egg:
1 - shiny shell; 2 - polar bodies; 3 - blastomeres; 4 - light blastomeres forming trophoblast; 5 - dark blastomeres; 6 - trophoblast; 7 - germinal nodule.
Rice. 63. Scheme of movement of a splitting cow zygote along the oviduct.
The cavity of the blastocyst is filled with fluid. It was formed as a result of the absorption of uterine gland secretions by trophoblast cells. Initially, the blastocyst is free in 6h uterine cavity. Then, with the help of villi formed on the surface of the trophoblast, the blastocyst attaches to the wall of the uterus. This process is called implantation (im - penetration into, plantatio - planting) (Fig. 66). At the large cattle Implantation occurs on the 17th day, in the horse on the 63rd - 70th day, in the macaque - on the 9th day after fertilization. Then the cells of the germinal node line up in the form of a layer - a germinal disk is formed, similar to the germinal disk of birds. In its middle part, a compacted zone is differentiated - the embryonic shield. As in birds, the body of the embryo develops from the material of the embryonic shield, and the rest of the embryonic disc is used in the formation of provisional organs.
Thus, despite the fact that in higher mammals, due to the secondary loss of yolk, the eggs are oligolecithal with holoblastic cleavage, the structure of the blastula is similar to that which is formed after meroblastic cleavage. This can be explained by the fact that the predecessors of mammals had polylecithal, telolecithal eggs and higher mammals inherited the structure of the blastula from their ancestors, the latter resembles the blastula of birds.
Gastrulation. Formation of axial organs and their differentiation. Gastrulation occurs in the same way as in reptiles, birds, and lower mammals. By delamination of the germinal disc, ectoderm and endoderm are formed. If these leaves were formed from the material of the germinal scutellum, then they are called germinal, and if they arose from the non-embryonic zone of the germinal disc, then they are not germinal. Non-embryonic ectoderm and endoderm grow along the inner surface of the trophoblast. Soon the trophoblast located above the embryo is resorbed and the latter ends up lying for some time in the uterine cavity, uncovered.
Rice. 64. Scheme of ovulation, fertilization, crushing, implantation:
1 - primordial follicles; 2 - growing follicles; 3, 4 - vesicular follicles; 5 - ovulated egg; 6 - collapsed vesicular follicle; 7 - yellow body; 8 - fimbriae of the oviduct funnel; 9 - the egg at the moment of sperm penetration into it; 10 - sperm; 11 - zygote, pronuclei bringing together; 12 - zygote in metaphase; 13 - splitting up; 14 - morula; 15 - blastocyst; 16 - implantation.
The formation of mesoderm proceeds in the same way as in birds. The cells of the marginal zone of the discoblastula migrate in two streams to the posterior part of the embryo. Here these flows meet and change their direction of movement. Now they move forward in the center of the germinal disk, forming the primary streak with a longitudinal depression - the primary groove. At the anterior end of the primary stripe, a Hensen's node with a depression - the primary fossa - is formed. In this zone, the material of the future notochord is tucked in and grows forward between the ectoderm and endoderm in the form of a head (chordal) process (Fig. 67).
Mesoderm develops from the cells of the primitive streak. After migration, its material grows between the ectoderm and endoderm and turns into segmented mesoderm (somites), adjacent segmental legs and unsegmented mesoderm. Somites consist of a sclerotome (ventromedial part), a dermotome (lateral part), and a myotome (medial part). Somites can connect to unsegmented mesoderm through segmental stalks. The unsegmented part of the mesoderm has the appearance of a hollow sac. Its outer wall is called the parietal layer, and the inner wall is called the visceral layer. The cavity enclosed between them is called the secondary body cavity, or coelom (Fig. 68).
Rice. 65. Fragmentation of the zygote and formation of the pig blastocyst:
A - G- successive stages of crushing (black- - blastomeres, from which the body of the embryo will develop; white- blastomeres from which the trophoblast will develop); D- blastocyst; E - AND- development of the germinal disc and formation of endoderm; TO- formation of mesoderm and primary gut from endoderm; 1 - germinal nodule; 2 - trophoblast; 3 - blastocoel; 4 - shiny zone; 5 - endoderm cells; 6 - endoderm; 7 - germinal disc; 8 - ectoderm of the germinal disc; 9 - trophectoderm; 10 - mesoderm; 11 - primary gut (wall) (according to Patten).
Rice. 66. Macaque embryo at the age of 9 days at the time of implantation:
1 - embryoblast; 2 - part of the trophoblast that penetrates into the tissue of the uterus; 3 - 5 - uterine tissue (3 - epithelium, 4 - basis of the mucous membrane; 5 - gland in a state of dystrophy) (according to Vislotsky, Streeter).
The differentiation of the germ layers proceeds in the same way as in birds and other animals. On the dorsal part of the embryo, a neural plate is formed in the ectoderm; after its edges fuse, the neural tube is formed. The ectoderm grows on it, so very soon the neural tube becomes submerged under the ectoderm. The entire nervous system develops from the neural tube, and the superficial layer of skin (epidermis) develops from the ectoderm. The notochord does not function as an organ in adult animals. It is completely replaced by the vertebrae of the spinal column. Somite myotomes are the source of the formation of the trunk muscles, and sclerotomes are the mesenchyme, from which bone and cartilage tissue then develop. Derma-tom - the rudiment of the deep layers of the skin
Rice. 67. Rabbit embryo, top view:
1 - head process; 2 - Hensen's knot; 3 - primary fossa; 4 - primary stripe.
Rice. 68. Cross section of a mammalian embryo at the 11-segment stage. Visible connection with the uterus:
1 - uterine glands; 2 - visceral and 3 - parietal layers of mesoderm; 4 - myotome; 5 - aorta; 6 - intraembryonic coelom; 7 - extraembryonic coelom; S- endoderm of the yolk sac; 9 - chorionic villi; 10 - trophoblast; 11 - ectoderm.
cover. The urinary and reproductive systems are formed from the material of the segmental legs, which is why it is called nephrogonadotomy.
The superficial tissue (epithelium) of the parietal layer of the pleura and peritoneum is formed from the parietal layer of the splanchnotome, and the epithelium of the serous membranes of those organs that lie in the thoracic and abdominal cavities is formed from the visceral layer.
Epithelium develops from the endoderm, covering inner surface digestive tube and organs - derivatives of the digestive tube: respiratory organs, liver, pancreas.
Thus, the development of germ layers and their further differentiation in mammals is similar to those in other animals. These signs are the most ancient; they reflect the path that mammals have traveled in their development. Such characteristics are classified as palingenetic (palin - again, genesis - birth) in contrast to coenogenetic, that is, acquired in connection with changes in living conditions, for example, the emergence of animals from water to land.
Not only the permanent organs of the embryo develop from the germ layers - ectoderm, endoderm and mesoderm. They participate in the laying of temporary, or provisional, organs - the membranes.
Formation of extraembryonic (temporary) organs(Fig. 69). It is believed that one of the features of the development of mammals is that during the isolecithal egg and holoblastic fragmentation, the formation of temporary organs occurs. As is known, in the evolution of chordates, provisional organs are the acquisition of vertebrates with telolecithal, polylecithal eggs and meroblastic cleavage.
Rice. 69. Scheme of development of the yolk sac and embryonic membranes in mammals (six successive stages):
A - the process of fouling of the amniotic sac cavity with endoderm (1) and mesoderm (2); IN- formation of a closed endodermal vesicle (4); IN - the beginning of the formation of the amniotic fold (5) and intestinal philtrum (6); G- separation of the body of the embryo (7); yolk sac (8); D- closure of amniotic folds (9); beginning of formation of allantois development (10); E- closed amniotic cavity (11); developed allantois (12); chorionic villi (13); parietal layer of mesoderm (14); visceral layer of mesoderm (15); ectoderm (3).
Another feature of the development of mammals is the very early separation of the embryonic from the non-embryonic part. Thus, already at the beginning of crushing, blastomeres are formed, forming an extra-embryonic auxiliary membrane - the trophoblast, with the help of which the embryo begins to receive nutrients
Rice. 70. Diagram of the relationship between the uterus and the yolk sac in a rabbit:
1 - allantoic placenta; 2 - yolk sac; 3 - wall of the uterus; 4 - amnion.
substances from the uterine cavity. After the formation of the germ layers, the trophoblast located above the embryo is reduced. The unreduced part of the trophoblast, merging with the ectoderm, forms a single layer. Adjacent with inside To this layer, sheets of unsegmented mesoderm and extraembryonic ectoderm grow.
Simultaneously with the formation of the embryo's body, the development of the fetal membranes occurs: the yolk sac, amnion, chorion, allantois.
The yolk sac, as in birds, is formed from the extraembryonic endoderm and the visceral layer of mesoderm. Unlike birds, it does not contain yolk, but a protein liquid. Blood vessels form in the wall of the yolk sac. This membrane performs hematopoietic and trophic functions. The latter comes down to the processing and delivery of nutrients from the mother’s body to the embryo (Fig. 70,71). The duration of yolk sac function varies from animal to animal.
As in birds, in mammals the development of membranes begins with the formation of two folds - the trunk and the amniotic. The trunk fold lifts the embryo above the yolk sac and separates its embryonic part from the non-embryonic part, and the embryonic endoderm closes into the intestinal tube. However, the intestinal tube remains connected to the yolk sac by a narrow vitelline stalk (duct). The tip of the trunk fold is directed under the body of the embryo, while all the germ layers bend: ectoderm, unsegmented mesoderm, endoderm.
The formation of the amniotic fold involves the trophoblast, fused with the extraembryonic ectoderm and the parietal layer of mesedermis. The amniotic fold has two parts: internal and external. Each of them is built from leaves of the same name, but differs in the order of their arrangement. So, the inner layer of the inner part of the amniotic fold is the ectoderm, which in the outer part of the amniotic fold will be on the outside. This also applies to the sequence of occurrence of the parietal layer of mesoderm. The amniotic fold is directed above the body of the embryo. After its edges have fused, the embryo becomes surrounded by two membranes at once - the amnion and the chorion.
Rice. 71. Scheme of migration of primary germ cells from the yolk sac to the gonad primordium ( different stages migrations are conventionally plotted on the same cross section of the embryo):
1 - epithelium of the yolk sac; 2 - mesenchyme; 3 - vessels; 4 - primary kidney; 5 - gonad primordium; 6 - primary germ cells; 7 - rudimentary epithelium.
The amnion develops from the inner part of the amniotic fold, the chorion - from the outer part. The cavity that forms around the embryo is called the amniotic cavity. It is filled with a transparent watery liquid, in the formation of which the amnion and the embryo take part. Amniotic fluid protects the embryo from excessive loss of water, serves as a protective environment, softens shocks, creates the possibility of embryo mobility, and ensures the exchange of amniotic fluid. The amnion wall consists of extraembryonic ectoderm directed into the amnion cavity and the parietal layer of mesoderm located outside the ectoderm.
The chorion is homologous to the serosa of birds and other animals. It develops from the outer part of the amniotic fold, and is therefore built from a trophoblast connected to the ectoderm and a parietal layer of mesoderm. On the surface of the chorion, processes are formed - secondary villi, growing into the wall of the uterus. This zone is greatly thickened, abundantly supplied with blood vessels and is called the baby's place, or placenta. The main function of the placenta is to supply the embryo with nutrients, oxygen and free its blood from carbon dioxide and unnecessary metabolic products. The flow of substances into and out of the blood of the embryo is carried out diffusely or through active transfer, that is, with the cost of this process
Rice. 72. Scheme of relationships between organs in the fetus of animals with epitheliochorial type of placentation:
1 - allanto-amnion; 2 - allanto-chorion; 3 - chorionic villi; 4 - cavity of the urinary sac; 5 - amnion cavity; 6 - yolk sac.
energy. However, it should be noted that the mother’s blood does not mix with the blood of the fetus either in the placenta or in other parts of the chorion.
The placenta, being an organ of nutrition, excretion, and respiration of the fetus, also performs the function of an organ endocrine system. Hormones synthesized by the trophoblast and then by the placenta ensure the normal course of pregnancy.
There are several types of placenta based on their shape.
1. Diffuse placenta (Fig. 72) - its secondary papillae develop over the entire surface of the chorion. It is found in pigs, horses, camels, marsupials, cetaceans, and hippopotamus. Chorionic villi penetrate the glands of the uterine wall without destroying the uterine tissue. Since the latter is covered with epithelium, according to its structure this type of placenta is called epitheliochorial, or hemiplacenta (Fig. 73). The embryo is nourished in the following way - the uterine glands secrete royal jelly, which is absorbed into the blood vessels of the chorionic villi. During childbirth, the chorionic villi move out of the uterine glands without tissue destruction, so there is usually no bleeding.
2. Cotyledon placenta (Fig. 74) - the chorionic villi are located in bushes - cotyledons. They connect to thickenings of the uterine wall, which are called caruncles. The cotyledon-caruncle complex is called the placentome. In this zone, the epithelium of the uterine wall dissolves and the cotyledons are immersed in a deeper (connective tissue) layer of the uterine wall. Such a placenta is called desmochorial and is characteristic of artiodactyls. According to some scientists, ruminants also have an epitheliochorionic placenta.
3. Belt placenta (Fig. 75). The zone of chorionic villi in the form of a wide belt surrounds the amniotic sac. The connection between the embryo and the uterine wall is closer: chorionic villi are located in the connective tissue layer of the uterine wall, in contact with the endothelial layer of the wall blood vessels. This. The placenta is called endotheliochorionic.
4. Discoidal placenta. The contact area between the chorionic villi and the uterine wall has the shape of a disc. The chorionic villi are immersed in blood-filled lacunae lying in the connective tissue layer of the uterine wall. This type of placenta is called hemochorionic and is found in primates.
Allantois is an outgrowth of the ventral wall of the hindgut. Like the intestine, it consists of endoderm and a visceral layer of mesoderm. In some mammals, nitrogenous metabolic products accumulate in it, so it functions like a bladder. In most animals, due to the very early development of the embryo with the maternal organism, the allantois is developed much less well than in birds. Blood vessels from the embryo and placenta pass through the wall of the allantois. After blood vessels grow into the allantois, the latter begins to take part in the metabolism of the embryo.
The junction of the allantois with the chorion is called the chorioallantois or allantoic placenta. The embryo is connected to the placenta through the umbilical cord. It consists of a narrow duct of the yolk sac, allantois and
Rice. 73. Scheme of placentas:
A- epitheliochorial; b- desmochorial; V- endotheliochorial; G- hemochorial; 1 - chorion epithelium; 2 - epithelium of the uterine wall; 3 - connective tissue chorionic villi; 4 - connective tissue of the uterine wall; 5 - blood vessels of the chorionic villi; 6 - blood vessels of the uterine wall; 7 ~ maternal blood.
Rice. 74 Amniotic sac with the fetus of a cow at the age of 120 days:
1 - cotyledons; 2 - umbilical cord.
blood vessels. In some animals, the Et yolk sac is associated with the placenta. This type of placenta is called yolk placenta.
Thus, the duration of embryogenesis varies in different placental animals. It is determined by the maturity of the birth of the babies and the nature of the connection between the embryo and the mother’s body, that is, the structure of the placenta.
Embryogenesis of farm animals proceeds similarly and differs from primates. These developmental features will be briefly discussed below.
In obstetric practice, intrauterine development is divided into three periods: embryonic (fetal), prefetal and fetal. The embryonic period is characterized by the development of characteristics typical of all vertebrates and mammals. During the prefetal period, the characteristics characteristic of this family are laid down. During the fertile period, species, breed and individual structural features develop.
In cattle, the duration of intrauterine development is 270 days (9 months). According to G. A. Schmidt, the germinal (embryonic) period lasts the first 34 days, the pre-fertal period - from the 35th to the 60th day, the fetal period - from the 61st to the 270th day.
During the first week, the zygote is fragmented and the trophoblast is formed. The embryo is nourished by the yolk of the egg. In this case, oxygen-free breakdown of nutrients occurs.
From the 8th to the 20th day is the stage of development of the germ layers, axial organs, amnion and yolk sac (Fig. 76). Nutrition and respiration are carried out, as a rule, with the help of trophoblast.
On the 20th - 23rd day, the trunk fold develops, the digestive tube and allantois are formed. Nutrition and respiration occur with the participation of blood vessels.
24 - 34 days - the stage of formation of the placenta, chorion cotyledons, and many organ systems. Nutrition and respiration of the embryo
Rice. 75. Zonar (belt) placenta of carnivorous animals.
Rice. 76. Cow embryo at the stage of closure of the neural tube ridges (age 21 days):
1 - neural plate; 2 - general structures of skeletal muscles and skeleton; 3 - laying of the allantois.
Rice. 77. Cross section of a 15-day-old primate embryo at the level of the primitive streak:
1 - plasmodiotrophoblast; 2 - cytotrophoblast; 3 - connective tissue of the chorion; 4 - amniotic leg; 5 - amnion ectoderm; 6 - outer layer of the embryonic shield; 7 - mitotically dividing cell; 8 - endoderm; 9 - mesoderm of the primitive streak; 10 - amniotic cavity; 11 - cavity of the yolk sac.
carried out through the vessels of the allantois connected to the trophoblast.
35 - 50 days - early pre-fetal period. During this period, the number of cotyledons increases, the cartilaginous skeleton and mammary gland are formed.
50 - 60 days - the late pre-fetal period, characterized by the formation of the bone skeleton, the development of signs of the animal's sex.
Rice. 78. Scheme of a sagittal section of a 3-week human embryo:
1 - cutaneous ectoderm; 2 - amnion ectoderm; 3 - amnion mesoderm; 4 - intestinal endoderm; 5 - vitelline endoderm; 6 - chord; 7 - allantois; 8 - rudiments of the heart; 9 - blood islands; 10 - amniotic leg; 11 - chorion; 12 - chorionic villi.
61 - 120 days - early fetal period: development of breed characteristics.
121 - 270 days - late fetal period: formation and growth of all organ systems, development individual characteristics buildings.
In other species of farm animals, the periods of intrauterine development have been studied in less detail. In sheep, the embryonic period occurs during the first 29 days after fertilization. The prefetal period lasts from the 29th to the 45th day. Then comes the fertile period.
The duration of the periods of intrauterine development of pigs differs from cattle and sheep. The embryonic period lasts 21 days, the prefertal period lasts from the 21st day to the beginning of the second month, and then the fertile period begins.
Embryogenesis of primates is characterized by the following features: there is no correlation in the development of the trophoblast, extraembryonic mesoderm and embryo; early formation of the amnion and yolk sac; thickening of the trophoblast lying above the embryoblast, which helps to strengthen the connection between the embryo and the maternal body.
Trophoblast cells synthesize enzymes that destroy uterine tissue and the germinal vesicle, plunging into them, comes into contact with the mother’s body.
From the expanding endoderm, which is formed by delamination of the embryoblast, the yolk vesicle is formed. The ectoderm of the embryoblast splits. In the cleavage zone, a first insignificant and then rapidly expanding cavity is formed - the amniotic sac (Fig. 77).
The area of the embryoblast bordering the vitelline and amniotic sacs thickens and becomes a two-layer embryonic shield. The layer facing the amniotic sac is the ectoderm, and the layer facing the yolk sac is the endoderm. In the embryonic shield, the primary streak with Hensen's node is formed - the sources of development of the notochord and mesoderm. The outside of the embryo is covered with trophoblast. Its inner layer is the extraembryonic mesoderm, or the so-called amniotic leg. The allantois is located here. The latter also develops from the intestinal endoderm. The vessels of the allantois wall connect the embryo with the placenta (Fig. 78).
Further stages of embryogenesis in primates proceed in the same way as in other mammals.
All mammals feed their young with milk. It contains all the substances necessary for the development of the body and is very easily digestible. Depending on the characteristics of reproduction and development, mammals are divided into three groups.
Oviparous or primal beasts
Oviparous animals, or primal beasts, do not give birth to live young, but lay eggs. In addition, like reptiles and birds, they have a cloaca. These include the platypus and echidnas, which live in Australia and the surrounding islands.
Figure: Oviparous mammals - platypus, echidna
Platypus- a completely unique animal. It is the size of a rabbit, and in front it has a horny protrusion, similar to a duck's beak (hence its name). When the stuffed animal was first brought to Europe, scientists mistook it for a fake and decided that the duck’s beak was sewn onto some kind of animal. Everyone was even more amazed when it turned out that the platypus... lays eggs and incubates them! Who is he: a bird or a mammal? However, it turned out that after hatching its cub still feeds on milk. The mammary glands do not have nipples. Therefore, milk, like sweat, is secreted onto the fur, from which the cub licks it off.
Echidnas in appearance they somewhat resemble a hedgehog with very long spines. They also lay eggs, but do not incubate them, but carry them in a pouch on their belly. Just like the platypus, the cubs lick the milk secreted over the entire surface of the abdomen.
The body temperature of oviparous birds ranges from 25-30 °C.
Marsupial mammals
Marsupials are mammals whose babies are born very small, weak and helpless (for example, a 2 m tall kangaroo has a baby born only 3 cm long). That's why for a long time The mother carries such a baby in a pouch on her belly. The bag contains mammary glands with nipples. A newborn baby usually hangs on the nipple without letting it out of its mouth. As he grows up, he begins to crawl out of the pouch and eat the same food as adult animals. However, for a long time, in case of danger, the cub hides in the pouch and is again reinforced with milk, although at this time its younger brother may already be hanging on the other nipple.
Drawing: Marsupial mammals- kangaroo, marsupial mouse, koala
Marsupials are widespread in Australia and America. In total, about 270 species are known. Among them, the most famous are kangaroos. They move by jumping on hind legs, and the front legs serve only to move grass and branches towards the mouth.
Placental mammals
Placentals are mammals in which a small egg, after fertilization, develops in a special organ - uterus, and the embryo attaches to the wall of the uterus placenta. In the placenta through umbilical cord there is close contact between the blood vessels of the mother and the fetus. From the mother's blood, the embryo receives all the necessary nutrients and oxygen, and metabolic products are released back into the mother’s blood.
Figure: Carrying an embryo inside the body placental mammals
The process of intrauterine development of the embryo of viviparous animals and humans is called pregnancy. Gestation periods vary among mammals. As a rule, in small animals it is short (for example, in some mouse-like rodents it is 11-15 days), in medium-sized animals it is several months, in large animals it is a year or more. In addition, lifestyle affects these timings. Those that give birth to cubs in burrows, hollows and other shelters have a short pregnancy. Their cubs are born blind, helpless, numbering 5-6 in medium-sized animals and 8-12 in small ones. Those animals that do not live hiding in holes and moving quickly have a long pregnancy. The cubs of such mammals are born large, well-developed, and within a few hours they can follow their mother. Their number, due to their large size, is only 1-2.
The frequency of reproduction is also related to the size of the animal and the timing of pregnancy: the shorter the pregnancy, the more often reproduction is repeated. So, small mouse-like rodents can have 5-8 litters a year, large ones reproduce once every few years.
Feeding babies with milk
Feeding babies with milk is one of the most characteristic features all mammals (hence the name of this class). Milk is produced in the female's mammary glands, which are usually located on the chest or belly. The ducts of the mammary glands open outward through small holes at the end of the nipples, the number of which varies (from 2 to 22) and depends on the fertility of the species. A domestic dog that produces 3-8 puppies has 8 teats.
Milk has a very high nutritional value and contains all the necessary substances for the growth and development of babies: water, fats, proteins, carbohydrates, vitamins and mineral salts. White color milk depends on the fact that the fat included in its composition has the form of microscopically small droplets. Such fat is easily digested and absorbed in the baby’s body.
At first, the female feeds her offspring with only milk. Grown-up cubs switch to regular food.
164) Look at the drawing. Label the names of the parts skin mammals, indicated by numbers.
I - epidermis
II - the skin itself
2) sebaceous gland
3) sweat gland.
165) What sense organs do mammals have?
Organs of touch - skin receptors; olfactory organ - nasal cavity; organ of taste - tongue; organ of vision - eyes; organ of hearing - ear.
1660 Study the table “Class Mammals. The structure of a rabbit." Look at the drawing. Write the names of the bones of the mammalian skeleton, indicated by numbers.
2) cervical vertebrae
3) thoracic vertebrae
4) caudal vertebrae
5) pelvic bones
9) chest
10) forearm
13) shoulder blade.
167) List the bones that make up the shoulder and pelvic girdle of mammals.
Shoulder girdle: paired shoulder blades and collarbones.
Pelvic girdle: paired iliac, ischial and pubic bones.
168) List the structural features of the skeleton associated with a terrestrial lifestyle.
1) The appearance of full-fledged limbs - paws built on the principle of levers ending in a hand with tenacious fingers - ensure effective movement on land. Belts of the limbs appear, and muscles are attached to them to ensure the movement of the paws.
2) Appearance cervical spine spine - allows you to move your head in different directions, which contributes to better orientation in space
3) Bones become tubular - this gives increased strength and at the same time lightens the skeleton
4) Development of the jaw apparatus. Both herbivores and carnivores have a need for more careful processing of food. In this regard, differentiated teeth appear.
5) The number of cervical vertebrae is constant and equal to seven, the skull is more voluminous, which is associated with the larger size of the brain. The bones of the skull fuse quite late, allowing the brain to grow as the animal grows.
6) The limb is five-fingered. The methods of movement of mammals are different - walking, running, climbing, flying, digging, swimming - which is reflected in the structure of the limbs.
169) What are the structural features of the mammalian brain?
The mammalian brain has the same parts as the brains of other vertebrates, but is larger in size and very complex structure forebrain hemispheres. Their outer layer consists of nerve cells, forming the cerebral cortex. It is in the cerebral cortex that the main processes of higher nervous activity occur. In more highly organized species of mammals, the cerebral cortex forms numerous convolutions and grooves, which sharply increases its area. The cerebellum and midbrain are well developed, since mammals are characterized by high physical activity and complex reflexes. The sense organs are distinguished by great complexity and perfection.
170) Study the table “Class Mammals. The structure of a rabbit." Look at the drawing. Write the names internal organs rabbit, indicated by numbers.
4) stomach
6) bladder
7) large intestine
8) small intestine
9) diaphragm
171) What is a diaphragm? What are its functions?
The diaphragm is an unpaired vastus muscle that separates the pectoralis and abdominal cavity, which serves to expand the lungs. Conventionally, its border can be drawn along the lower edge of the ribs. Formed by a system of striated muscles. Characteristic only of mammals.
172) Fill the table.
Mammalian organ systems.
Mammalian internal organ system Organs Functions Muscular muscles, diaphragm active lifestyle and getting around Sense organs eyes, ears, skin and whiskers, tongue, nasal cavity relationship with the environment Digestive system oral cavity, pharynx, esophagus, stomach, duodenum, intestines, rectum, anus digestion Respiratory system nasal cavity, larynx, trachea, bronchi, alveolar lungs gas exchange Circulatory system four-chambered heart, vessels (arteries, veins, capellars) oxygen transport Excretory system kidneys, ureter, bladder, urethra removal of harmful substances Reproductive system testes, vas deferens/ovaries, uterus, vagina reproduction of one's own kind
173) Describe the function of the mammalian kidney.
The kidneys of mammals consist of an outer (cortical) and inner (medullary) layer. In the cortex there are convoluted tubules originating from Bowman's capsules, inside which there are glomeruli of blood vessels. The filtration process is carried out in them, and blood plasma is filtered into the renal tubules - primary urine is formed. The renal tubules form several knees, in which water, sugar and amino acids are reabsorbed from the primary urine - secondary urine is formed, which enters the collecting ducts, which form the medulla. The final product of protein metabolism is urea. Urine enters the ureters, then into the bladder and then through the urethra to the outside.
174) Draw a diagram of the structure of the mammalian heart and label its components.
175) Using the picture in the textbook on p. 236, describe how blood moves through blood vessels in mammals.
The pulmonary circulation begins in the right ventricle through the pulmonary trunk. Venous blood travels through the pulmonary trunk through the pulmonary arteries to the lungs. Enriched with oxygen in the lungs, the blood returns through the pulmonary veins to the left atrium, and from there enters the left ventricle.
The systemic circulation begins with the aorta, which emerges from the left ventricle. From there, the blood enters large vessels heading to the head, torso and limbs. Large vessels branch into small ones, which pass into intraorgan arteries, and then into arterioles and capillaries. Through capillaries, constant exchange of substances occurs between blood and tissues. The capillaries unite and merge into venules and veins, which merge into large venous vessels, forming the superior and inferior vena cava. Through them, blood returns to the right atrium.
176) What blood enters the right atrium?
Deoxygenated blood.
177) Look at the drawing and sign it. Describe how the baby develops in the uterus.
Fertilization is internal and occurs in the oviducts. During development, the placenta is formed in the uterus, through which a connection is established between the embryo and the mother’s body. As a result, gas exchange in the body of the embryo, its nutrition and removal of metabolic products are ensured. The duration of pregnancy depends on many factors: body size, readiness of the offspring for independent life etc. In some species, the cubs are born helpless, in others - capable of active action.
178) What is the placenta? What is its biological significance?
The placenta is an embryonic organ in all female placental mammals, some marsupials and a number of other groups of animals, allowing for the transfer of material between the circulatory systems of the fetus and mother.
1) Gas exchange
2) Excretory
3) Hormonal
4) Protective.
179) What is the structure of the reproductive system of mammals?
The reproductive system in males consists of paired testes, vas deferens, accessory glands and a copulatory organ. The testes (in which sperm are formed and mature) in most species are located in a special sac - the scrotum.
In females reproductive system consists of paired ovaries, oviducts, uterus and vagina. Oocytes are produced in the ovaries. As they mature, they are released and first enter the upper sections of the oviducts, where, as a rule, they are fertilized. The fertilized egg moves to the uterus, where, with further development of the embryo, it is formed children's place- placenta.
180) Provide evidence that mammals evolved from ancient reptiles.
Mammals have many similarities with reptiles, especially in embryonic development, skeletal structure, and horny integument. This suggests that mammals evolved from ancient reptiles. The similarity of adult animals with reptiles is also manifested in the abundance of horny skin formations in animals (fur, horns, hooves, claws, nails); presence of scales on the tails of mice, rats, beavers, similar to the scales of reptiles; in the similar development of these scales from the skin rudiments. The similarity with reptiles is especially noticeable in the primitive egg-laying primates - the platypus and the echidna.
Ancient toothed reptiles had a great resemblance to mammals. They were called that because they, like mammals, had differentiated teeth (incisors, canines and molars). In the process of evolution, mammals have developed such survival adaptations as perfect thermoregulation, constant body temperature, viviparity and feeding their young with milk. Particularly important was the improvement of the central nervous system: strong development of the forebrain hemispheres in connection with complex behavior, improvement of the senses. This contributed to the wide geographical distribution of mammals and their development of all habitats, the emergence various forms bodies, high numbers.
181) Is it true that the first animals are closer to reptiles than other mammals? Why?
Right. In Australia and on the surrounding islands they live oviparous mammals, which in their structure and reproductive characteristics occupy an intermediate position between reptiles and mammals. These include the primal beasts: the platypus and the echidna.
Similarities with reptiles:
1) When breeding, they lay eggs covered with a durable shell that protects the contents of the egg from drying out. (But the echidna carries a single egg in a special pouch, which is a fold of skin on the ventral side of the body).
2) The intestines and urogenital openings open into the cloaca.
3) There are no nipples. (But there are mammary glands).
4) The shoulder girdle is similar to that of reptiles.
5) Low temperature bodies.
6) The jaws are covered with a horny beak.
182) Name the representatives of marsupials. What is characteristic of them?
Marsupials: kaola, kangaroo, marsupial wolf, opossum.
Traits: no placenta, cubs are born underdeveloped and very small, and are borne in a pouch; the brain is very primitive; the temperature is lower than that of placentals and is not constant.
183) Name the main characteristics of placental mammals, which indicate their higher organization compared to proto-beasts and marsupials.
Placental, or higher beasts- the most common infraclass of mammals, considered the most highly developed. Distinctive feature placental is birth in a relatively developed stage. This is possible due to the presence of the placenta, through which the embryo receives nutrients and antibodies from the mother and gets rid of waste products. Their embryo develops in the mother's uterus, is covered by the placenta and feeds and breathes through the umbilical cord.
Placentals have a very well developed brain, especially the forebrain (gyri, cortex cerebral hemispheres) and cerebellum, typical challenging behavior, caring for offspring.
184) Which orders include: shrew, fruit bat, lynx, hippopotamus?
The shrew is a shrew order.
Krylan - order Chiroptera.
Lynx - carnivorous squad.
Hippopotamus - order Artiodactyls.
185) Complete the laboratory work “Structure of Mammals”, presented in the notebook for laboratory work and independent observations “Biology. Diversity of living organisms. 7th grade".
Among many animals asexually Only so-called invertebrate animals reproduce. Vertebrates - such as mammals, fish, reptiles, birds and amphibians - reproduce sexually: sperm and eggs, carrying hereditary material typical for a given animal species, are united during fertilization. A fertilized egg is called an embryo.
Depending on the species of animal, the embryo can develop both inside and outside the mother's body. Gradually, small cubs develop from fertilized eggs in accordance with the genetic instructions embedded in it. Many, such as frogs, go through one more stage of development before becoming fully grown.
From egg through larva to adult animal
Snails live on land, in running water and in the seas. Sea slugs lay eggs in sea water, which after high tide get stuck between the rocks. From fertilized eggs, larvae (veligers) emerge that can swim. They swim with the current and finally sink to the rocky bottom, where they develop into adult crawling clams.
Fertilized egg
The red spot in the middle of the egg yolk is a three-day-old chicken embryo. After a week, the embryo already takes the shape of a chicken. After a month, the chicken is already fully developed and covered with delicate down. With the egg tooth on his beak, he breaks the egg shell and comes out into the light. The chick hatches and becomes an adult without any additional developmental stage.
From egg to tadpole
During mating season, many frogs gather in large, noisy groups. Females respond to loud calls from males. Only a few species of frogs give birth to live young; most species lay eggs (spawn) in or near water. The number of eggs depends on the type of frog and ranges from one to twenty-five thousand. Typically, the eggs are fertilized outside the frog's body and left to fend for themselves. When the egg matures, a small tadpole hatches from it. Tadpoles live in water and breathe through gills, like fish. In only a few species of frogs, females care for their young.
Frogs and Toads
Unlike adult frogs, tadpoles are herbivorous and feed on aquatic plants and algae. After a certain time, an amazing transformation (metamorphosis) occurs in the development of the tadpole: fore and hind limbs appear, the tail disappears, lungs and eyelids develop, and new systems digestion, designed to digest animal food.
The conversion rate is different for different types, the main factor here is water temperature. In some toads and frogs, metamorphosis occurs in a few days or weeks, while in others it takes several months. The tadpole of the North American bullfrog takes a year or more to fully develop.
Frogs and toads belong to the class of amphibians and to the same group of tailless amphibians, but they differ appearance and way of life. Frogs have soft skin and are good jumpers, while toads are covered in warts and tend to crawl. There are more than 3,500 species of frogs and toads on earth. With the exception of Antarctica, they can be found on all continents. They prefer to live in tropical and subtropical zones, where more than 80% of all species live. But no matter where they live, in deserts or mountains, savannas or tropical rain forests, they must return to water to procreate.
What is metamorphosis
In their development, frogs go through three stages: from egg to tadpole, and then to adult frog. This developmental process is called metamorphosis. Many invertebrates also go through the larval stage in their development. However, the most amazing changes occur in the lives of insects: butterflies and beetles, flies and wasps. Their life is divided into four stages, very different from each other in their feeding method and habitat: egg, larva, pupa, adult insect. The larva looks completely different from the adult insect and does not have wings. Her life is completely focused on growth and development, and not on procreation. Only after the larva pupates does it become an adult insect.