The developing embryo is provided with many special structures which protect the embryo from physical injury. These structures are present in all the embryos whether they are developing inside or outside the body.
Protective Coats of Embryos
These are special structures to protect the embryo. These structures are in the form of membranes or coats, it can be best seen by studying a developing chick (birds) embryo.
Protection By Internal Development
In most of the mammals development takes place in the reproductive tract (uterus). It provides a safe and moist environment for the development of the embryo.
In some lower vertebrate e.g. sharks and some reptiles, the embryo develops in the uterus. There is no biological contact between the tissue of the mother and the embryo. The nutrition for the developing embryo is provided by the egg. The uterus provides only the safe environment for the developing embryo.
The mammals are divided into three sub classes:
Prototheria: These are eggs laying mammals e.g. Duckbill platypus. Duckbill platypus produces an egg very much like the egg of a reptile. The egg is not laid immediately after it is formed. It is retained for a time in the body of the female. Then it is laid in a nest and hatched. With the hatching, the mammary glands of the mother secrete milk. The young ones are fed on the mammary glands. In this way the neonates i.e. the new born get the food. This is a very important step in the care of the young ones by the mammals.
Metatheria: The pouched mammals. e.g. Opossum and Kangaroos etc. The eggs are small having little yolk. The yolk is only enough for the development of very small and immature individuals. Early development occurs within the body of the female. Then the tiny, almost helpless infants emerge out of the female's body. They crawl up the female's belly and slip into the pouch. Here the infants are sheltered and kept warm. There are mammary glands in the pouch. Each infant, feeds on the mother's milk as it continues its development. The young ones stay in the pouch till they grow and become independent of the mother.
Eutheria: The placental mammals e.g. Cow, Goat, Man etc. A placenta is a structure that develops between the developing embryo and uterus of the female. Exchange of nutrition and waste materials takes place through the placenta. The eggs have a negligible amount of yolk. So the female provides the needed food materials to the embryo through the placenta. The young ones when born are fed on the milk secreted by the mammary glands. Thus the placental mammals provide all possible nourishment and protection to the embryos.