THIRD PERIOD OF DEVELOPMENT. 123 



The lumen of the now closed neural canal is at first 

 dorsiventral and elliptically elongated, and does not, 

 till later, become circular in transverse section. The 

 transverse section of the whole neural canal in its form 

 continually reminds one of the earlier stages (Figs. 116, 

 118), while its dorsal edges still have a wide lateral 

 extension. It is only gradually, as we shall see in later 

 stages, and through shortening of these cells (Fig. 117 

 is very instructive), that the transverse section of the 

 neural canal becomes spherical (Figs. 125, 128). Its 

 transverse section is at first more or less trapezoidal 

 (Figs. 116, 118). The ventral side is a little deepened 

 into the shape of a channel, and this channel is in 

 contact with the dorsal surface of the notochord. This 

 quadrilateral form is at once altered, owing to the 

 diminution of the dorsal surface. The transverse 

 section of the neural canal is now spherical, with a 

 ventral segment for the upper surface of the notochord 

 (Figs. 126-128). 



The closing of the neural canal does not follow from 

 front to back, as would be conjectured according to the 

 differentiation which in general moves in this way, 

 but the shrinking together and closing, succeed at the 

 hinder end somewhat earlier, and advance towards the 



