3l6 THE EVOLUTION OF MAN. 



the back. The closing is effected in exactly the same way 

 as that of the spinal tube, which is now entirely enclosed 

 by the vertebral canal. In this way the dorsal wall is 

 formed, and the spinal tube lies quite in the interior (Fig. 

 98). In the same way the primitive vertebral mass grows 

 lower down round the notochord {chorda dorsalis), there 

 forming the vertebral column. In this lower part the inner 

 under edge of the primitive vertebral plates on each side 

 splits into two laminae, the upper of which passes in 

 between the notochord and the spinal tube, while the under, 

 on the contrary, penetrates between the notochord and the 

 intestinal tube. These two laminae, by meeting from each 

 side above and below the notochord, completely enclose the 

 latter, and thus form the tubular outer notochord-sheath, 

 the skeleton-forming layer, from which the vertebral 

 column arises (Figs. 97, 98). (Cf. Figs. 3-6 on Plate IV., 

 and the following chapter.) 



Processes similar to these which take place above, on 

 the back, during the formation of the dorsal wall, are 

 observed below, on the abdomen, during the formation of 

 the ventral wall (Fig. 98, bh). Here the side-plates grow 

 together round the intestine in a similar way to that in 

 which the intestine itself closed. The outer part of the 

 side-plates forms the ventral wall, or the lower body- wall, 

 while on the inner side of the amnion-fold, which has been 

 mentioned, the two side-plates curve more and grow toward 

 each other from right and left. While the intestinal canal 

 is closing, the closing of the ventral wall is also taking place 

 from all sides. Thus the ventral wall, which encloses the 

 whole ventral cavity below, also originates from two halves, 

 from the two side-plates, which incline toward each other ; 



