ON THE PROCESS OP GÂSTRULA.TION IN CHELONIA. 255 



for the present tlie invagination-cavity (B) to be the Arche n teron . 

 Then the yolk reservoir A must from the nature of the 

 thing be an appendag-e of the in vagi nation-cavity B. 

 Hut owing to its enormous size compared witli B, the bodily invagina- 

 tion of tlie yolk is out of the tpiestion. It forms the most conspicuous 

 part of the egg from tlie first, and begins to surround itself with a 

 cell-layer long before the invaginntion-cavity B makes its appear- 

 ance even. Hence, A can only secondarily come into connection with 

 B. This ha])pens by the anterior part of the floor of B flaring out, 

 so to speak, into a funnel-shaped opening. This is the mean- 

 ing of the breaking through of the invagination-cavity. The spread- 

 ing of the cells derived from the ])rimitive knob over the ventral 

 surface of the embryonic shield after the breaking through of the 

 invao'ination-cavitv mav be regarded as the gradual Iv thinnino- wall of 

 the funnel-shaj^ed opening making itself continuous with tlie cell-layer 

 surrounding the yolk (Woodcut III, o-longitudin;d section, 4-cross 

 section). When the alimentary canal is formed definitely, later on in 

 the course of development, the wall of the funnel-shaped opening of 

 the invagination cavity is again tucked in as the splanchnopleura. 



The above course of reasoninu* explains all the events accompany- 

 ing the invagination and thus justifies the assumption that the in- 

 vagination-cavity (B) \fi the Archen ter on corresponding to the part 

 marked as such in the amphibian egg (Woodcut II.) and the whole 

 yolk-bag must be regarded simply as a part of its ventral wall that 

 has become bulged out on account of the enormous accumulation of 

 nutritive matter within it. The jiresence of a large subgerminal 

 cavity in the yolk filled with a nutritive li(piid is a physiological 

 accident, so to speak. I agree with Van Beneden, Keibel, and 

 Wenckebach in regarding it as intercellular space in the yolk. 

 It is a cavity arisen solely from physiological necessity 



