DEVELOPMENT OF THE NERVOUS SYSTEM. 



35 



and bony fishes this is accomplished by the neural plate rolling 

 or folding up so that the neural folds meet above. The folds begin 

 to fuse together at a point which later falls in the region of the 

 mesencephalon or farther caudally, and the fusion continues 

 forward and backward until a complete tube, the neural tube, is 

 formed. The tube remains open for some time at the anterior 

 end; the opening is called the neuropore. At the caudal end the 

 tube remains for some time in connection with the archenteron 

 by way of the neurenteric canal. The extent and relations of the 

 neuropore will be important in later connections ; the neurenteric 

 canal need not claim further attention. In the mean time the 

 general ectoderm fuses over the dorsal surface of the neural tube 



FIG. 13. Sections of the neural plate and folds in amphibia. A, the cephalic 

 portion of the neural plate in Amblystoma tigrinum. The neural folds are more 

 darkly shaded. It is evident that they are actual folds of the ectoderm. B, one 

 side of the neural plate of Amblystoma punctatum at a slightly more advanced 

 stage of development andat a higher magnification. 



and when the neuropore closes the tube lies wholly beneath the 

 surface of the body. In cyclostomes and bony fishes the same 

 results are reached by the neural plate thickening and sinking 

 down as a solid cord of cells which comes to lie beneath the ecto- 

 derm. Within the cord of cells appear clefts which unite into a 

 continuous canal. 



Roughly speaking the neural tube goes to form the spinal cord, 

 brain and motor nerves. During its development, the material 



