THE DEVELOPMENT OF THE FROG 257 



surface, and the appearance of a broad but shallow median 

 groove, the so-called primitive groove. The flattened surface 

 is the neural plate. Its edges soon become thicker, and 



Fig. 60 



A. Dorsal view of a frog's egg, showing the neural folds and the blastopore lying 

 between their posterior ends. S. A somewhat later stage seen from in front and 

 above, showing the sense plate and the anterior ends of the neural folds dilated 

 to form the brain. C Dorsal view of a later embryo in which the neural fold's 

 are completely closed, and five mesoblastic somites are indicated externally. 

 3fi, blastopore ; /d, fore-brain ; ^/, gill-plates ; meSy mesoblastic somites ; nf, 

 neural folds ; jc, spinal cord ; j/, sense plate. 



are raised up to form the neural folds, which are continued 

 into each other anteriorly in front of the primitive groove, 

 but posteriorly diverp-e somewhat so as to embrace the 



II. R 



Digitized by Microsoft® 



