650 ZOOLOGY. 



develops in a specialized portion of the oviducts, the uterus 

 or womb, and that the growing germ until birth is supplied 

 not with yolk as food, but 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 1827 

 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 being less than a quarter 

 of a millimetre ( T ^ 7 inch) in diameter. 



The mammalian embryo, nourished as it is through the 

 maternal circulation, needs additional temporary organs ; 

 these are the chorion (Fig- 541, ck), formed from the vitelline 

 membrane (present in birds as well as mammals), 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 effete 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 proto vertebrae 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 placenta has developed 

 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 



