44 Jenkinson, Germinal Layers of Vertebrates. 



front it consists of a single layer of cubical cells ; behind 

 the dorsal lip it is thickened — the primitive plate — and 

 from the thickening there proceeds backwards a narrow 

 tongue of cells between the upper layer and the paraderm. 

 A transverse section through the blastopore shows the 

 mass of cells of the primitive plate flanked on each side 

 by a projecting blastoporic lip and sending out between 

 the upper layer and the paraderm two lateral sheets of 

 cells {Fig. 26, A). 



h-i A 



Fig. 26. 



A series of four transverse sections through the blastopore and 

 archenteron of Platydactyliis (after Will). 



A. Posterior section showing yolk-plug {y.p.) and mesoderm {mes.) 

 springing from the lateral lips (/./.). Underneath is the paraderm (p.d,). 



B. Section in front of the yolk-plug but behind the dorsal lip. 



C. Section in front of the dorsal lip ; the floor of the archenteron is 

 fused with the paraderm. 



D. The floor of the archenteron, together with the underlying 

 paraderm, has come away, and the archenteric and subgerminal cavities are 

 in communication with one another. 



The resemblance between these structures — ignoring 

 for the moment the paraderm — and those seen in the 

 Amphibian egg when the blastopore has become circular 

 is sufficiently obvious. The dorsal lip and the lateral 

 lips (there is no ventral lip in the Reptiles) clearly 

 correspond in the two cases ; the mass of cells in the 

 primitive plate embraced by these lips is the yolk plug ; 

 the cavity of invagination is the archenteron in which 



