THE NERVOUS SYSTEM 



119 



them deepens, and the folds becoming more and more prominent 

 bend in towards each other (Fig. 26) and finally meet and fuse, 

 thereby converting the neural groove into a tube. 



The neural folds first meet about the junction of the head 

 and trunk of the future tadpole, from which point the fusion 

 extends rapidly in both directions, forwards and backwards. 

 The last point at which fusion occurs is a little distance behind 

 the anterior end of the tube, at the place where the pineal body 

 will appear later. 



In front, the neural tube ends blindly ; at its posterior end 



NC 



/ 



.BH 



M 



FIG. 27, Longitudinal vertical section through a frog embryo 

 shortly before the closure of the blastopore. Length of the embryo 

 2 '5 mm. x 30. 



B, blastopore; BF, fore-brain; BH, hind-brain; BM, mid-brain; 

 H, hypoblast ; L, liver ; M, mesoblast ; MN, mesenteron ; N, noto- 

 chord ; NC, neurenteric canal ; P, ingrowth of epiblast to form pituitary 

 body ; PD, proctodaeum ; R, rectal diverticulum of mesenteron ; S, cen- 

 tral canal of spinal cord ; Y, yolk-cells. 



it opens to the exterior at the blastopore, and is in free com- 

 munication with the mesenteron. (Cf. Fig. 27.) The short 

 channel of communication between the neural tube and the 

 mesenteron, i.e., between the nervous system and the alimen- 

 tary canal, is spoken of as the neurenteric canal ; it is only 



