Spinal Canal in Chick Embryos. 



153 



Up to 42 hours the cord is unclosed and normal. At 44 

 hours, though the cord is closed in the lumbar region, there is 

 no sign of any unusual development. At 48 hours the first sign 

 of subdivision is seen in the form of a marked thickening and 

 projection inwards of the wall on one side of the cord, the 

 cavity, however, not being completely subdivided, although some- 

 what reduced. 



Fig. ]. > 



Text figure I. represents diagramatically five consecutive trans- 

 verse sections of the cord at 48 hours' incubation. The projec- 

 tion (p) into the central canal (c) is well shown. 



At 50 hours two distinct cavities are present, as seen in the 

 figures. The projection may be seen to have grown completely 

 across the cord in this region in such a way as to divide off the 



* 1 Fig. 2. 3 -^ 



single central canal into two unequally-sized canals, Cj and c.^ 

 (see text fig. II., 2 and 3) ; this inequality of the resulting 

 canals having been already suggested in the earlier stage by the 

 peculiar shape of the internal projection of the wall of the cord. 

 The total absence of any canal at the most posterior end of the 

 cord is shown in section 1, fig. II. 



