MEDULLARY CANAL. 



give rise on the one hand to the formation of the central nervous 

 system, and on the other to the formation of the notochord and 

 mesoblastic somites 1 . The former structure is developed from 

 the epiblast and the two latter from the hypoblast. 



The formation of the central nervous system commences 

 with the flattening of the dorsal surface of the embryo. The 

 flattened area forms a plate (fig. 2 B and fig. 3 A, up), extending 

 backwards to the blastopore, which has in the meantime passed 

 round to the dorsal surface. The sides of the plate become 

 raised as two folds, which are most prominent posteriorly, and 

 meet behind the blastopore, but shade off in front. The two 

 folds next unite dorsally, so as to convert the previous groove 

 into a canal 2 the neural or medullary canal. They unite first 

 of all over the blastopore, and their line of junction extends 

 from this point forwards (fig. 2 C, D, E). There is in this way 

 formed a tube on the floor of which the blastopore opens behind, 

 and which is itself open in front. Finally the medullary canal 

 is formed for the whole length of the embryo. The anterior 

 opening persists however for some time. The communication 

 between the neural and alimentary tracts becomes interrupted 

 when the caudal fin appears and the anus is formed. The 

 neural canal then extends round the end of the notochord to the 

 ventral side, but subsequently retreats to the dorsal side and 

 terminates in a slight dilatation. 



In the formation of the medullary canal there are two points 

 deserving notice viz. (i) the connection with the blastopore; 

 (2) the relation of the walls of the canal to the adjoining 

 epiblast. With reference to the first of these points it is clear 

 that the fact of the blastopore opening on the floor of the neural 

 canal causes a free communication to exist between the archen- 

 teron or gastrula cavity and the neural canal ; and that, so long 

 as the anterior pore of the neural canal remains open, the 

 archenteron communicates indirectly with the exterior (vide 

 fig. 2 E). It must not however be supposed (as has been done 

 by some embryologists) that the pore at the front end of the 

 neural canal represents the blastopore carried forwards. It is 



1 The protovertebrse of most embryologists will be spoken of as mesoblastic 

 somites. 



" The details of this process are spoken of below. 



