34 Vertebrate Embryology 



folds begins in what will be the neck region of 

 the future tadpole, and extends forward and 

 backward from that point, the extreme anterior 

 end being the last to close in completely. As 

 the neural folds, or medullary folds as they 

 are often called, fuse together to form the- 

 neural or medullary canal, the epidermal layer 

 of ectoblast fuses over the top, and forms, once 

 more, a smooth, continuous layer (Figs. 12 

 and 15). 



It will be seen by an examination of the 

 figures that the chief thickness of the wall of 

 the neural tube is derived from the nervous 

 layer of ectoblast, but that the layer of cells 

 which lines the inside of the tube is from the 

 epidermal layer (Fig. 13). 



The extreme posterior ends of the neural 

 folds extend on each side of the blastopore, so 

 that, as they come together, they cover the 

 blastopore, which persists for a time as a nar- 

 row passage, the neurenteric canal, connecting 

 the mesenteron with the neural canal (Fig. 

 \2, NC 2Si^ NT). 



The neural tube, formed as above described, 

 becomes converted into the central nervous 

 system, the anterior end forming the brain, the 

 rest of the tube forming the spinal cord. 



