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. 

 e further processes 

 development take 

 ,ce principally in 

 plate. Its most 

 perficial cells are 

 flattened out to thin 

 scales, such as also 

 brm the wall of the 

 tula elsewhere ; its 

 aining elements, 

 the contrary, ar- 

 in from two 

 three superposed 

 layers, are larger and 



I her in protoplasm. 

 dp to this time the 

 bryo 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. -Rabbits egg, 70-90 hours after fertilisation, after 

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

 Embryology." 



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

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



Fig. 60. Cross section through the almost circular germinal area of a Rabbif a 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 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 



