LOPMENT OP i in: i wo I'IMM \KV .KHM I A 



LOS 



One circumstance i> .-j.ecially charact<vi>t ic of the g.-tstnil i 

 Mammals: that the invaginat ing 

 membrane is not a closed blind sac, 

 but pis>'N>os a free margin, with 

 which it gr<iw> :il>ng mi the inner 

 Mu-face of tho outer germ-layer, 

 until it has completely lined tho 

 blastoderm ic vesicle. The reader 

 will please compare with this the 

 si at. ments on page 102. But the 

 absence of a ventral closure becomes 

 intelligible, when wo imagine that 

 the yolk-mass, which constitutes in 

 meroblastic eggs or in Amphibian 

 the floor of the coelenteron, 

 has degenerated and wholly disap- 

 peared. In this case crelenteron 

 and cleavage-cavity become one 

 and the game, as is the case with 

 Mammals. 



Moreover we are induced to as- 

 sume that in the eggs of Mammals a 

 ivirivive metamorphosis of origin- 

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

 many phenomena in their development, which would be unintelligible 



x^ 



Fig. 63. Fear-shaped embryonic spot of a 

 Rabbit's egg 6 days and 18 hours old, 

 after KOLLIKER. 



ps, Short primitive streak ; hw, crescent- 

 shaped terminal ridge ; V, anterior, 

 H, posterior end. 



Fig. 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, nft.T HKAPE. 



pore; ak, outer, iX-, iiuifi -im -1 :iy.-r ; 1", .-interior, //, posterior end. 



without this assumption. These phenomena will bo con>idei\d more 

 at length in a subsequent chapter. 



