412 THE DEVELOPMENT OF THE CHICK 
notochord is destined to be completely replaced by the bodies of 
the vertebra, derived from the sclerotomes. 
The Sclerotomes and Vertebral Segmentation. The vertebral 
segmentation does not agree with the primitive divisions of the 
somites, but alternates with it; or in other words, the centers 
of the vertebrae do not coincide with the centers of the original 
somites, but with the intersomitiec septa in which the segmental 
arteries run. Thus each myotome extends over half of two 
vertebral segments, and the spinal ganglia and nerves tend to 
alternate with the vertebra. It therefore happens that each myo- 
tome exerts traction on two vertebra, obviously an advantageous 
arrangement, and the spinal nerves lie opposite the intervertebral 
foramina. 
This arrangement is brought about by the development of 
each vertebra from the caudal half of one sclerotome and the 
cephalic half of the sclerotome immediately behind; parts of 
two somites enter into the composition of each vertebra, as is 
very obvious at an early stage: Fig. 232 represents a section 
through the base of the tail of a chick embryo of ninety-six hours; 
it is approximately frontal, but is inclined ventro-dorsally from 
behind forwards. The original somites are indicated by the 
myotomes and the segmental arteries. In the region of the 
notochord one can plainly distinguish three parts to each 
sclerotome, viz., (1) a narrow, median, or perichordal part 
abutting on the notochord, in which no divisions occur either 
within or between somites; (2) a caudal lateral division distin- 
guished by the denser aggregation of the cells from (3) the cephalic 
division. Between the caudal and cephalic divisions of the sclero- 
tome is a fissure (intervertebral fissure) which marks the boundary 
of the future vertebre. Each vertebra in fact arises from the 
caudal component of one sclerotome and the cephalic component 
of the sclerotome immediately behind. Between adjacent sclero- 
tomes is the intersomitie septum containing the segmental artery. 
If one follows these conditions back into successively earlier stages, 
one finds that the intervertebral fissure arises from the primitive 
somitie cavity, and that the distinction between caudal and 
cephalic divisions of the sclerotome is marked continuously from a 
very early stage by the presence of the intervertebral fissure and 
the greater density of the caudal division, 7.e., the cephalic com- 
ponent of each definitive vertebra. 
