Germinal Layers 



41 



which lie between the outer layer and the yolk, and their arrange- 

 ment into a definite membrane, an inner sheet of tissue is pro- 

 duced, forming eventually the gut epithelium. 



This, at any rate, is the condition attained after a few hours 

 of incubation and as soon as this has been accomplished we can 

 recognise the presence of what are known as the primary germinal 

 layers, and the process of gastrulation has been finished. Gastrula- 

 tion is simply the formation of a cavity which is the future gut 

 cavity, and the lining of this cavity by a definite layer of cells, 

 the future epithelium of the true gut or mesenteron. 



There is as yet, Fig. 23, no opening to the exterior, no mouth 

 nor anus. 



Fig. 23. A, Bird in oviduct; B, bird laid; C, bird after short incubation. 

 ec ectoderm; end endoderm; sg segmentation cavity. 



The outer membrane about this period becomes dissociated 

 from the underlying layer, at any rate at its margin, and it 

 extends beyond it and creeps slowly over the yolk. 



The names of these several layers and parts are as follows. 



The outer layer is called the ectoderm, the inner the endoderm. 



The endoderm is obviously continuous at its margin with the 

 general yolk mass, and is greatly thickened here. 



The thickened margin is known as the germinal wall, the 

 cavity as the subgerminal cavity, or archenteron. 



If we take the account just given of the origin of the gut 

 cavity in the bird's egg as correct, it will fit in very well with the 

 origin of the gut cavity in both the other great classes of Amniotes, 

 the Reptiles and Mammals. 



The condition of a solid morula is converted into the condition 

 represented in Fig. 24, by accumulation of fluid in spaces among 



