34 Jenkinson, Germinal Layers of Vertebrates. 



closes bilaterally by an overgrowth of the usual character, 



but there is no ventral lip {Fig. 19, A) ; on this side 



the area of small animal cells is extended by delami- 



nation from the yolk. Dorsally the mesoderm is carried 



into place during the overgrowth of the blastoporic lip ; 



ventrally it is differentiated from the vegetative cells 



{Fig. 19, B). The notochord is split off the roof of the 



archenteron (which then becomes the gut), as in the frog, 



which this type indeed closely resembles in all respects, 



except in the amount of yolk and the absence of the 



ventral lip. 



AMPHIBIA. 



Urodeld. 



In the Urodela, the process of germ-layer formation 



Fig. 20. 

 Formation of the germ-layers in the Axolotl. 

 (Original.) 

 A."]^ Sagittal section at the time of closure of the blastopore. 



B. Transverse section of the same stage. Letters as before. 



C. Transverse section at a later stage, to show the conversion of the 

 whole of the archenteric roof into notochord. The lateral yolk-cells are 

 growing in towards the middle line. 



