THIRD PERIOD OF DEVELOPMENT. 89 



lying underneath them, so that a flat neural canal 



v, 

 is constructed underneath the outer covering, but 



still wide open (Figs. 79-81). This opens outwards 

 more widely, in the region in front of the first 

 mesoblastic somite. 



This opening diminishes but slowly in the later 

 stages, and that too while advancing from back to 

 front. The result of this is that the close of this 

 opening may be viewed as a very lengthened con- 

 tinuation of the process of closing the neural canal 

 which we have just observed. 



The cavity between neural plate and outer cover- 

 ing is thus a narrow slit, which in front, in the 

 part before the first mesoblastic somite, opens out- 

 wards. Behind, owing to the remnant of the 

 gastrula-mouth continuing in existence, it stands in 

 communication with the lumen of the archenteron 

 as a neurentic canal. 



The hollow space, at first narrow, which we will 

 now call the neural canal, becomes deeper for two 

 reasons. The first is that the neural plate in the 

 course of the development shrinks closer. The second 

 is that the dorsal covering, which, even after the 

 arching of the neural canal is completed, is still 



