THE EARLY DEVELOPMENT OF THE MAMMAL 381 



telolecithal, probably like that of the Sauropsida, and we are 

 already familiar with the fact that the presence of a large yolk- 

 mass profoundly modifies the simple processes of cleavage, 

 gastrulation, etc. Accompanying the return of the mammalian 

 egg to the nearly alecithal condition, however, we do not find a 

 corresponding return to the simpler early developmental 

 processes characteristic of the primarily alecithal egg. On the 

 contrary these processes are modified in new directions, not 

 known elsewhere. 



Added to these modifications is another series of alterations 

 resulting from the very early development of a mechanism by 

 which the segmenting ovum becomes intimately related with 

 the uterine walls. Altogether, then, we find conditions here 

 that are very special and not closely paralleled in other animals. 

 So profoundly have the early stages of development become 

 modified that there is sometimes difficulty in clearly identifying 

 and homologizing with other' types, certain structures of the 

 early mammalian embryo, such as the germ layers, primitive 

 streak, etc. In the following description we shall adopt the 

 convenient and more customary descriptive terms, but it 

 should be clearly recognized that there are possible contradic- 

 tions, and some students of mammalian embryology would say, 

 positive errors, in applying the customary terminology to the 

 early history of the mammalian embryo. 



In most of the Mammals the early development is very slow. 

 The cleavage stages usually occur as the egg is passing slowly 

 down the oviduct. In the mouse the first and second cleavages 

 occur about twenty-four hours and forty-eight hours, respec- 

 tively, after coitus, and about eighty hours elapse before the 

 ovum reaches the uterus. In the rabbit the first cleavage 

 occurs fourteen to fifteen hours after ovulation, and nearly 

 four days are occupied in the passage to the uterus. In the 

 dog eight to ten days elapse before the ovum reaches the uterus. 

 A few instances are known, on the other hand, where the egg 

 arrives in the uterus just as cleavage begins (bat, hedgehog, and 

 other Insectivors). In the European roe-deer, where fertiliza- 

 tion occurs in the autumn, or perhaps in mid-summer, the ovum 



