DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 



101 



of cells, which projects into the cleavage-cavity, has become meta- 

 morphosed and has spread itself out more and more in the 

 form of a disc-like plate, which is continuous at its attenuated 

 margins with the thin 

 wall of the blastula. 

 The further processes 

 of development take 



place principally in 

 this plate. Its most 

 superficial cells are 

 flattened out to thin 

 scales, such as also 

 form the wall of the 

 blastula elsewhere ; its 

 remaining elements, 

 on the contrary, ar- 

 ranged in from two 

 to three superposed 

 layers, are larger and 

 richer in protoplasm. 



Up to this time the 

 embryo of the Mammal 

 is in the blastula stage. 

 It still consists everywhere of a single germ -layer. For the view 

 which has been advanced by many persons, that the germ-disc in this 



Fig. 59. Rabbit's egg, 70-90 hours after fertilisation, after 

 ED. v. BENEDEN. Copied from BALFOUB'S "Comparative 

 Embryology." 



bv, Cavity of the blastula ; zp, [gelatinous layer surrounding 

 the] zona pellucida ; ep, hy, as in Fig. 58. 



Fig. 60. Gross section through the almost circular germinal area of a Rabbif egg 6 days and 9 



hours old (diameter 0*8 mm.), after BALFODR. 

 <nk, Outer, ik, inner germ-layer. The section shows the peculiar character of the upper layer with 



a certain number of flattened superficial cells. Only about half of the whole breadth of the 



germinal area is represanted. 



stage of development is already in the two-layered condition, and that 

 the outer layer of flat cells constitutes the outer germ-layer and the 

 more protoplasmic cells lying under it the inner germ-layer, is, in my 

 opinion, untenable. Opposed to this are, first, the fact that the flat- 

 tened and the thicker cell-layers are firmly joined together and 

 are not separated from each other even by the narrowest fissure, 

 and, secondly, the further course of the development.* 



* Holding to this interpretation, I am of course also unable to agree with a 

 view of VAN BENEDEN'S, according to which the gastrulation takes place at the 



