414 THE DEVELOPMENT OF THE CHICK 



Now, if one follows these components as they appear at suc- 

 cessively higher levels in such a frontal section as Fig. 232, one 

 finds that the perichordal layer disappears in the region of the 

 neural tube, and that the spinal ganglia appear in the cephalic 

 division of the sclerotome, and almost completely replace it. 

 Thus the caudal division of the sclerotome is more extensive, as 

 well as denser, than the cephalic division. 



In transverse sections one finds that the sclerotomic mesen- 

 chyme spreads towards the middle line and tends to fill all the 

 interspaces between the notochord and neural tube, on the one 

 hand, and the myotomes on the other. But there is no time at 

 which the sclerotome tissue of successive somites forms a con- 

 tinuous unsegmented mass in which the vertebral segmentation 

 appears secondarily, as maintained by Froriep, except in the thin 

 perichondal layer; on the contrary, successive sclero tomes and 

 vertebral components may be continuously distinguished, except 

 in the perichordal layer; and the fusion of caudal and cephalic 

 sclerotome halves to form single vertebrae may be continuously 

 followed. Thus, although the segmentation of the vertebrae is 

 with reference to the myotomes and ganglia, it is dependent 

 upon separation of original sclerotome halves, and not secondarily 

 produced in a continuous mass. 



Summarizing the conditions at ninety-six hours, we may say 

 that the vertebrae are represented by a continuous perichordal 

 layer of rather loose mesenchyme and two mesenchymatous 

 arches in each segment, that ascend from the perichordal layer 

 to the sides of the neural tube; in each segment the upper part 

 of the cephalic sclerotomic arch is occupied almost completely 

 by the spinal ganglion, but the caudal arch ascends higher, though 

 not to the dorsal edge of the neural tube. The cranial and caudal 

 arches of any segment represent halves of contiguous, not of the 

 same, definitive vertebra. 



Membranous Stage of the Vertebrae. In the following or 

 membranous stage, the definitive segmentation of the vertebrae 

 is established, and the principal parts are laid down in the 

 membrane. These processes are essentially the same in all the 

 vertebrae, and the order of development is in the usual antero- 

 posterior direction. As regards the establishment of the verte- 

 bral segments: Figs. 233 and 234 represent frontal sections 

 through the same vertebral primordia at different levels from 



