178 EMBRYOLOGICAL TYPES 



the roof of the archenteron, which still forms a complete 



3, 



Fig. 77. — Transverse section through an embryo of Rana showing the 

 separation of the mesoderm (m) from the endodermal wall (en) of the 

 gut (g). 

 ec, ectoderm ; y, yolk-cells. Dorsally the ectoderm is thickening to 



form the neural folds (nf). 



Fig. 78. — Transverse section through an embryo of Rana slightly older than 

 the previous, showing the origin of the notochord (n) from the middle 

 line of the roof of the gut. The neural folds (nf) have risen up and 

 enclose a groove between them. 



covering to the archenteric cavity. This delamination of the 



