4-6 Jenkinson, Germinal Layers of Vertebrates. 



In other cases the resemblance may be just as striking ; 

 in the turtle, Trionyx, for example, the yolk-plug projects 

 in the characteristic Amphibian fashion i^Fig. 28). 



The floor of the archenteron now fuses throughout 

 with the paraderm below ; and as soon as the fusion is 

 completed perforations begin to appear in the fused 

 layers. They seem to be unable to keep pace with the 

 general growth of the blastoderm, and to become 

 first stretched and then fenestrated. But to whatever 

 causes the perforation may be due, the floor of the archen- 

 teron with the underlying paraderm completely disappears, 

 and the archenteron then communicates freely with the 

 sub-germinal cavity {Fig. 25, E). The roof of the archen- 

 teron is now inserted by its edges into the surrounding 

 paraderm {Fig. 27, B). This fusion of the archenteron 



^^I^^H^§- 





Fig. 28. 

 Transverse section of the blastopore of 7rionyx (after Mitsukuri). 

 Lettering as before ; the yolk is dotted. 



with the sub-germinal cavity is, as we shall have occasion 

 to see more fully later on, quite comparable to the com- 

 munication of the archenteron with the segmentation 

 cavity in the Gymnophiona ; the paraderm must then 

 be regarded as homologous with a part, but only a part, 

 of the yolk-cells in this group. 



The median strip of the roof next thickens to form 

 the notochord, and separates from the two lateral portions 

 which then become the mesoderm {Fig. 27, B, C, D). 

 These lateral plates pass posteriorly, of course, into the 



