DEVELOPMENT OF OTHER VERTEBRATES 433 



Birds and reptiles, therefore, produce eggs in which such a 

 large amount of yolk is deposited on one side of the ovum that the 

 active protoplasm, at the time of fertilization, becomes restricted 

 to a small disc at what was the animal pole of the egg cell at an 

 earlier stage of its formation. The great mass of yolk that is 

 thus formed is surrounded by an albuminous fluid which serves as 

 a cushion, and also as additional nutriment for the developing 

 embryo, and by a shell which is sufficiently impervious to the evap- 

 oration of water to enable the development to take place in the 

 atmosphere. It will be noted, however, that the embryo actually 

 develops in contact with fluids of the amniotic cavity and of the 

 other cavities enclosed within the egg shell. Terrestrial develop- 

 ment is possible because of these conditions, but at the same time 

 the immediate environment of the embryo is aquatic. 



Development of Mammals. — It is a well-known fact that the 

 more familiar mannnals develop within the body of the female 

 until they have reached the condition of miniature adults. Some 

 mammals, however, are born in a less developed state as, for 

 instance, the hairless young of rats and mice. More extreme exam- 

 ples are the young of opossums and kangaroos (Fig. 228), which 

 are born at a veiy early stage and complete their development in 

 the mai-supial pouch of the mother, with their mouths holding 

 fast to the nipples of the mammary glands. It is not so commonly 

 known that the most primitive order of mammals, now repre- 

 sented only by the duckbill or platypus, Ornithorhyncus anatinus 

 (Fig. 15, p. 29), and the spiny ant-eater. Echidna aculeata, develop 

 within eggs, which are encased in a shell and laid hke those of a 

 bird or reptile. The embryonic membranes in these mammals are 

 also hke those of birds and reptiles. There is, therefore, no stage 

 in their development comparable with the period of foetal life 

 within the mother that occurs in higher mammals. 



All other mammals develop within a part of the female genital 

 duct called the uterus, to the wall of which they are attached by a 

 structure known as the placenta (cf. Fig. 234, p. 450). The mam- 

 mal, of course, develops from an ovum, which is detached from the 

 ovary and enters the oviduct in the same general manner as the 

 eggs of birds and reptiles; but this egg has no large accumulation 

 of yolk, except in the two egg-laying mammals, the duckbill and 

 the spiny ant-eater. The ova are fertihzed by spermatozoa which 

 are introduced during sexual union and make their way to the 



