Where does the mammalian embryo develop? Features of mammals

Mammals (Grade 7 studies this topic in the section of zoology) unite all animals that feed their young with milk. Since this is the most numerous class of vertebrates, there are several classifications within a given systematic unit. One of them is based on where the mammalian embryo develops. This question will be discussed in detail in our article.

Types of mammals

All mammals are dioecious highly organized animals. Both females and males have paired sex glands: ovaries and testes, respectively. Outward they are opened by specialized ducts. For all these animals, only internal insemination is characteristic . However, on the basis of the development of the mammalian embryo, its further formation and level of development depends.

Who are the first-timers

Surprisingly, but true: there are mammals that lay eggs. They are called "pervozveri" and form only one detachment - One-pass. And he is named so because these animals have a cloaca. This is a primitive characteristic characteristic of previous chordates. The cloaca is one opening into which the products of the exchange of the urinary, digestive and reproductive systems are excreted.

Pervozveri very few, include only 6 species. Their place of residence is Australia with adjacent islands. Their representatives are echidna, proehidny and platypus. They do not have nipples, and milk through the ducts of the glands secrete on the surface of the skin. The cubs have to lick it off to get enough.

Very unusual is the appearance of these animals. Echidna is very similar to a large hedgehog. Its long and sharp needles are nothing more than a modification of the hairline characteristic of all mammals. Under them is a layer of dense wool. And if you take into account the fact that the front part of the muzzle is stretched into the proboscis, and the extremities are sharp claws, designed for digging, echidna and altogether remind a fantastic non-existent animal. Their females carry a single egg. They carefully store it in the fold of the skin, located on the ventral side of the body.

The platypus looks quite different. By its name, it is not difficult to guess that it has a wide and flat nose. On the fingers there are swimming membranes, and the tail has a flattened shape, acting like fins. Platypus eggs hatch their eggs in nests that suit themselves.

Subclass These beasts

Real beasts are united by the fact that they give birth to live young, and do not sit them out like birds. They already have no cloaca, and the ducts of the digestive, genital and urinary systems are opened by separate specialized openings. Depending on where the mammalian embryo develops, marsupials and placental mammals are distinguished.

Order Marsupials

They are also typical inhabitants of the Australian continent. They are characterized by the fact that the place where the mammalian embryo develops (egg), they leave very quickly. This is because they completely lack the placenta. As a result, cubs are born underdeveloped, small in size and incapable of independent life. After the birth of a certain time they are in a skin bag, also located on the ventral side of the body. It opens the ducts of the mammary glands.

The newborn cub is so weak and helpless that at first it can not even suck on its own. Muscles glands spontaneously contract and automatically sprinkle milk directly into the mouth of a small marsupial animal. Representatives of these animals are primarily kangaroos. They are able to move at a speed of about 60 km / h.

Quite common in Australia are the marsupial bear of koala and opossum.

Placental: where the mammalian embryo develops

The greatest development, and hence the spread of the planet, reached the mammalian animals that have the uterus, in which the placenta is formed during pregnancy. It binds the mother and baby throughout the pregnancy. Through the blood vessels of this formation, breathing and the exchange of nutrients are carried out. As a result, the future organism is provided with all vitally important conditions, and the walls of the uterus are protected from unfavorable environmental conditions. He is gaining enough strength to be quite viable immediately after birth and to move to independent existence after a short period of time. How progressive this feature is, says the number and the dominant position of animals that have this organ. Insectivores, Bats, Rodents, Hare-lobed, Artiodactyl and Ungulates, Predatory, Pinnipeds - this is not a complete list of orders that are placental. Yes, and the primates, to which the modern appearance of the Homo sapiens, have reached their development due to the presence of this sign including.

Features of mammals: advantages and disadvantages

Of course, the development of the embryo in the uterus with the help of the placenta is the most perfect way. When it forms, the uterine walls and the embryonic membrane tightly fuse. Through the placenta, all the necessary substances that are in the mother's body are transported to the body of the unborn child. In the womb, he is not afraid of temperature drops or predators. Naturally, the oviparous are in a less favorable position. This is an obvious fact. Of course, eggs contain a large number of nutrients necessary for the development of the embryo. But they can become a desirable prey for predatory animals. And given that the number of eggs 3-4 as much as possible, you can generally remain without offspring. The marsupials are born too undeveloped. For example, the duration of the process of pregnancy in opossum barely reaches two weeks. Naturally, in such a short period of time such a complex organism will not have time to achieve the proper degree of development.

Thus, the development of mammals has its own characteristics, which allow them to be grouped into three groups, each of which has its own adaptive features: single-pass, marsupial and placental.