DEVELOPMENT OF THE TWO PRIMARY GEUM-J.AYERS. 



105 



I, 



\ 



One circumstance is especially characteristic of the gastrulation of 



Mammals : that the invaginating 



membrane is not a closed blind sac, 



but possesses a free margin, with 



which it grows along on the inner 



\ surface of the outer germ-layer, 



i until it has completely lined the 



/ blastodermic vesicle. The reader 



will please compare with this the 



statements on page 102. But the 



absence of a ventral closure becomes 



intelligible, when we imagine that 



the yolk-mass^which constitutes in 



meroblastic eggs or in Amphibian 



eggs the floor of the ccelenteron, 



has degenerated and wholly disap- 

 peared. In this case ccelenteron 



and cleavage-cavity become one 



and the Same, as is the case with 



Mammals. 



Moreover we are induced to as- 

 sume that in the eggs of Mammals a 

 regressive metamorphosis of origin- 

 ally abundant yolk-contents must have taken place, on account of 

 many phenomena in their development, which would be unintelligible 



hio 



H 



. 63. Fear-shaped embryonic spot of a 

 Rabbit's egg 6 days and 18 hours old, 

 after KOLLIKER. 



Short primitive streak ; hw, crescent- 

 shaped terminal ridge ; V, anterior, 

 H, posterior end. 



ak 



Fig. 64. Median section of the embryonic fundament of a Mole's egg through that part in 



which the primitive streak has begun to be formed, after HEAPE. 

 u, Blastopore ; ak, outer, ik, inner germ-layer ; V, anterior, H, posterior end. 



without this assumption. These phenomena will be considered more 

 at length in a subsequent chapter. 



