DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 



105 



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, 

 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 

 nieroblastic 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 



' -' f~ -v r-^--. U 





liw 



- 



Fig. 63. Pear-shaped embryonic spot of a 

 Rabbit's egg 6 days and 18 hours old, 

 after KOLLIKER. 



pg, Short primitive streak ; hu\ crescent- 

 shaped terminal ridge ; V, anterior, 

 H, posterior end. 



li- 



ft I- 



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 ; ul-, 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. 



ik 



