24 Jenkinson, Germinal Layers of Vertebrates. 



of the roof of the archenteron ; posteriorly these sheets 

 pass into the caudal swellings — two thickenings at the 

 edge of the blastoderm, one on each side of the middle 

 line {Fig. 7, B, C ; Fig. 10, A) — where they fuse with 

 the roof of the archenteron, out of which they have 

 gradually been differentiated ; (2) the formation of 

 mesoderm is, however, not limited to the dorsal 

 h*p, but is carried on at the lateral lips, and, 

 as these extend forwards round the whole margin 

 of the blastoderm, at the anterior edge as well. 



c. 



Fig. II. 

 Two stages in the ventral closure of the gut in Scylluim. (Original.) 

 In B the ventral wall is seen to be formed by suture of the sides of the 

 archenteron : yolk and yolk-nuclei {y.n.) below. 



This extra embryonic mesoderm (' peristomales ' of 

 Rabl) is naturally continuous in the caudal swellings 

 with the embryonic ('gastrales ') mesoderm first described ; 

 it takes part only in the formation of the area vasculosa. 

 The notochord is formed from a median strip of cells 

 which is cut out of the roof of the archenteron (Fig. 10, 

 D, E ; Fig. II, A); the process begins in front and proceeds 

 backwards. The exact origin of the lining epithelium of 

 the alimentary canal — the endoderm — is by no means so 



