40 Jenkinson, Germinal Layers of Vertebrates. 



It appears that — 



(i) In all cases the edge of the blastoderm (applying 

 this term to both microlecithal and megalecithal 

 forms) becomes in whole or in part the lip of the 

 blastopore. In all cases the posterior edge be- 

 comes the dorsal lip, in all cases except the 

 Gymnophiona the anterior edge becomes the 

 ventral lip (unless this is absent). 



(2) The blastopore closes bilaterally ; the closure 



involves an overgrowth and ingrowth of cells 

 which is most vigorous at the dorsal, less vigorous 

 at the lateral, and least vigorous at the ventral 

 lip. 



(3) During this closure an archenteric cavity is formed 



and the material for the ectoderm, the notochord 

 and the mesoderm, and the roof of the archen- 

 teron is brought into its definitive position. The 

 notochord lies along the median line in front of 

 the dorsal lip, the lateral mesoderm sheets pass 

 into one another behind the ventral lip. All 

 three sets of cells ectoderm, notochord and meso- 

 derm, and roof of archenteron are continuous 

 with one another at the lips of the blastopore. 



(4) Distinct discrepancies are observable in the 



manner in which sets of cells which resemble 

 one another in origin are used in the production 

 of the germinal layers; this is particularly the 

 case with the roof of the archenteron, which 

 may form the whole, ox\ only a part of, or be 

 absolutely excluded from the definitive ali- 

 mentary tract. 



We may now turn to the Amniota. 



