50 EMBBYOLOGY OF THE LOWEE VEETEBEATES ch. 

 horse-shoe shaped gastrular lip corresponds to the yolk - plug of 



P:P 





4 , 1^:-'--;';'jt\ IgjSf^ 1 



^m 



end end' 



ent > 



ec? 



end 



Fig. 31. — Sagittal sections through early stages of l'latydactylus. (After Will, 1892.) 



A is of the stage shown in Fig. 29, A ; B is of the stage of Fig. 29, B ; B, C, and D are of intermediate 

 stages, e's, embryonic shield ; eel, ectoderm ; end', secondary endoderm ; ent, archenteric cavity ; 

 g.l, gastrular lip ; «M>, thickened ectoderm which will give rise later to the central nervous system 

 (medullary plate) ; p.p, primitive plate ; y.p, yolk-plug. 



amphibians and in some cases too (Laeerta — Will) it becomes com- 

 pletely enclosed, the tips of the horse-shoe curving inwards and 



