412 THE DEVELOPMENT OF THE CHICK 



notochord is destined to be completely replaced by the bodies of 

 the vertebrae, 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 intersomitic 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 vertebrae. It therefore happens that each myo- 

 tome exerts traction on two vertebrae, 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 vertebrae. 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 intersomitic 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 

 somitic 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, i.e., the cephalic com- 

 ponent of each definitive vertebra. 



