650 ZOOLOGY. 



develops in a siiecialized portion of the ovidnets, tlie uterus 

 or womb, and tliat tlio growing germ until birtli is sujiplied 

 not with yolk as food, bnt by the nourishment in the ma- 

 ternal blood. In fact, while the eggs of reptiles and birds 

 are enormous, it was not known with certainty until 1837 

 that mammals developed from eggs. The eggs of these an- 

 imals are very minute, owing in part to the minute amount 

 of yolk they contain ; that of man Ijeing less than a quarter 

 of a millimetre (^1-^ inch) in diameter. 



The mammalian embryo, nourished as it is through the 

 maternal circulation, needs additional temporary organs, 

 which the bird does not possess ; these are the chorion (Fig. 

 541, ch), a vascular layer of the allantois, which sends off 

 villi or processes extending into the walls of the womb. 

 Besides this, in the higher or placental mammals, the pla- 

 centa or after-birth is formed, which serves as an organ of 

 respiration as well as to supply the embryo or foetus with 

 nourishment, and to carry off its efEete products by means 

 of the maternal circulation. 



It is comparatively late in embryonic life that the mam- 

 malian features appear ; in the dog it is twenty-five days 

 before it can be told whether the embryo is a mammal or 

 not. 



All mammals may be said to pass through a morula and 

 gastrula stage. In the next stage when the nervous chord 

 and notochord arise, the mammalian germ is on the same 

 footing with an Ascidian larva. In a succeeding stage, 

 when the protovertebrse appear, an Amphioxus stage is 

 reached ; when a brain is formed, the level of the fishes is 

 reached ; after the limbs bud out the young mammals may 

 be said to assume the condition common to the embryos of 

 all Amphibian and higher Vertebrates. When the allantois 

 begins to appear the amphibian feature (the want of an 

 allantois) is dropped. When the chorion and placenta arise 

 the avian characters are surpassed and the mammalian feat- 

 ures assumed. Thus the development of the individual 

 mammal is an epitome of that of the branch or type to 

 which it belongs, and the successive steps in the degree of 

 specialization of the individual mammal are also paralleled 



