These considerations fully explain the characters of the secondary segmentation of the vertebral column. On the other hand the primary segmentation ([fig. 313]) of the vertebral rudiments is clearly a remnant of a condition when no vertebral bodies were present; and has no greater morphological significance than the fact that the cells of the vertebræ were derived from the segmented muscle-plates, and then became fused into a continuous sheath around the notochord and nervous axis; till finally they became in still higher forms differentiated into vertebræ and their arches.
During the stage represented in [fig. 28] g, and somewhat before the cartilaginous sheath of the notochord is formed, there appear four special concentrations of the mesoblastic tissue adjoining the notochord, two of them dorsal (neural) and two of them ventral (hæmal). They are not segmented, and form four ridges, seated on the sides of the notochord. They are united with each other by a delicate layer of tissue, and constitute the substance in which the neural and hæmal arches subsequently become differentiated.
Fig. 318. Section through the vertebral column of an advanced embryo of Scyllium in the region of the tail.
na. neural arch; ha. hæmal arch; ch. notochord; sh. inner sheath of notochord; ne. membrana elastic externa.
At about the time when the first traces of the cartilaginous sheath of the notochord arise, differentiations take place in the neural and hæmal ridges. In the neural ridge two sets of arches are formed for each myotome, and resting on the cartilaginous sheath of the notochord in the region which will afterwards form the centrum of a vertebra, and constituting a true neural arch; and a second separate from the cartilaginous sheath, forming an intercalated piece[200]. Both of them soon become hyaline cartilage.
There is a considerable portion of the original tissue of the neural ridge, especially in the immediate neighbourhood of the notochord, which is not employed in the formation of the neural arches. This tissue has a fibrous character and becomes converted into the perichondrium and other parts.
The hæmal arches are formed from the hæmal ridge in precisely the same way as the neural arches, but interhæmal intercalated pieces are often present. In the region of the tail the hæmal arches are continued into ventral processes which meet below, enclosing the aorta and caudal veins.
Since primitively the postanal gut was placed between the aorta and the caudal vein, the hæmal arches potentially invest a caudal section of the body cavity. In the trunk region they do not meet ventrally, but give support to the ribs. The structures just described are shewn in section in [fig. 318], in which the neural (na) and hæmal (ha) arches are shewn resting upon the cartilaginous sheath of the notochord.
While these changes are being effected in the arches the cartilaginous sheath of the notochord undergoes important differentiations. In the vertebral regions opposite the origin of the neural and hæmal arches ([fig. 318]) its outer part becomes hyaline cartilage, while the inner parts adjoining the notochord undergo a somewhat different development, the notochord in this part becomes at the same time somewhat constricted. In the intervertebral regions the cartilaginous sheath of the notochord becomes more definitely fibrous, while the notochord is in no way constricted. A diagrammatic longitudinal section through the vertebral column, while these changes are being effected, is shewn in [fig. 320] B.