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 



sp. 



Fig. 60 



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

 between their posterior ends. B '. 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 folds 

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

 bfi, blastopore ; /b, fore-brain ; gp, gill-plates ; mes, mesoblastic somites ; nf, 

 neural folds ; sc, spinal cord ',\sp, 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 diverge somewhat so as to embrace the 



II. R 



