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 



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

 Ep. v. BENEDEN. Copied from BALFOUR'S " Comparative 

 Embryology." 



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

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



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



hours old (diameter 0'8 mm.), after BALFOUR. 

 ak, 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 repressnted. 



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 



