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 sclerotomes and 
vertebral components may be continuously distinguished, except 
in the perichordal layer; and the fusion of caudal and cephalic 
sclerotome halves to form single vertebree may be continuously 
followed. Thus, although the segmentation of the vertebre 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-stx hours, we may say 
that the vertebre 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 seement the upper part 
of the cephalic selerotomic 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 vertebre 
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 
