20 Jenkinson, Germinal Layers of Vertebrates. 



those which remain in the yolk are probably concerned 

 solely with facilitating its liquefaction and absorption. 



There is now produced at the edge of the blastoderm 

 posteriorly and in the middle line a fold or overturning of 

 cells of the superficial layer. The fold, which is the dorsal 

 lip of the blastopore, is slightly raised and covers over a 

 space — the beginning of the archenteron — between itself 

 and the yolk. By the continuation of the backward growth 

 of the fold {Fig. 9, B, C) (the whole blastoderm is also 



Fig. 7. 

 Torpedo. A — C after Riickert, D — F after Ziegler (slightly modified). 



A. Formation of the dorsal lip at the posterior end of the blastoderm. 



B, C. Formation of the lateral lips (/./.) and caudal swellings (c.s.). 

 The extent of the archenteron, crescentic with a median prolongation, is 

 indicated by the shading. 



D, E, F. Extension of the blastoporic lip to the anterior edge of the 

 blastoderm. Formation of the embryo in the hinder region. 



Steadily spreading over the yolk) the archenteron attains a 

 considerable length ; its floor is formed of yolk into which 

 yolk-nuclei subsequently make their way ; its roof consists 

 of a columnar epithelium derived in part from the overturn- 

 ing of cells at the lip of the blastopore, in part possibly 

 from the posterior marginal lower layer cells. But while 



