30 JenkinsON, Germinal Layers of Vertebrates. 



as the ventral lip of the circular blastopore (yFig. 12) 

 {Fig. 13, D). The closure of the blastopore therefore 

 falls into the same two periods and takes place in 

 essentially the same manner as it does in the Elasmo- 

 branchs. 



Let us now return to the formation of the germ- 

 layers in the embryonic region. The superficial portion 

 of the blastoderm is the ectoderm. The endoderm, as 

 well as the notochord and mesoderm, is contained in 

 that sheet of cells — the roof of the archenteron — which 

 has grown forwards beneath the dorsal lip {Fig. 14, A). 

 In this sheet two narrow slit-like cavities presently 



Fig. 15. 

 Scnanus (after W^ilson). 

 Formation of the alimentary canal {al.c.) by fusion, and folding of the 

 two endodrrmal plates {end.) ; s.n.ch. sub-notochordal rod. 



appear, one on each side of the middle line ; each cavity 

 is parallel to the surface of the sheet, and soon extends 

 outwards to its edge ; the whole sheet is thus split on 

 each side with an upper and a lower plate, a median tract 

 remaining undivided {Fig. 14, B). This middle strip 

 then separates as the notochord from the two lateral 

 "portions. The latter consist now each of an upper meso- 



