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 

 )lastodermic vesicle. The reader 

 ,-ill please compare with this the 

 "atements on page 102. But the 

 ibsence of a ventral closure becomes 

 itelligible, when we imagine that 

 yolk-mass, which constitutes in 

 i?roblastic eggs or in Amphibian 

 s the floor of the coelenteron, 

 s degenerated and wholly disap- 

 a ared. In this case coelenteron 

 id cleavage-cavity become one 

 id the same, as is the case with 

 [animals. 



Moreover we are induced to as- 

 ime that in the eggs of Mammals a 

 >ive metamorphosis of origin- 

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

 lany phenomena in their development, which would be unintelligible 



ps 



hio 



Fig. 63. Pear-shaped embryonic spot of a 

 Rabbit's egg 6 days and 18 hours old, 

 after KOLLIKEB. 



ps, Short primitive streak ; hw, crescent- 

 shaped terminal ridge ; V, anterior, 

 H, posterior end. 



ik 



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

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

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



without this assumption. These phenomena will be considered more 

 it length in a subsequent chapter. 



