146 THE AMPHIOXUS. 



THE NEURAL CANAL. 



Tiie neural canal had become closed at the end 

 of the former period of development. It assumes a 

 rounded form on the transverse section with ventral 

 channel for the notochord. 



During the further elongation of the embryo the 

 transverse section of the neural canal becomes con- 

 tinually smaller, the canal diminishing chiefly in its 

 dorsal part, so that the ventral surface which is in 

 contact with the notochord is broadest. The edges 

 of the neural canal appear then as though drawn out 

 laterally (Figs. 134-140). 



The small lumen of the neural canal (primary 

 central canal) is spherical in the transverse section. 

 The cells which compose the canal surround this 

 cavity as a single layer. In the living object the 

 ciliation which has a posterior direction may always 

 be followed in the central canal. 



Since the elongation of the embryo takes place prin- 

 cipally in the region of the myotomes, in the same way 

 this is the region too in which the neural canal becomes 

 thinner. In the region of the unsegmented posterior 

 end it forms therefore a considerable swelling (Figs. 60, 



