CROSSOPTERYGIANS 



NEORHACHITOMES (TREMATOSAURS) 



BULLFROG 



LYRIOCEPHALUS 



COECILIANS 



Figure 6-18. The phylogeny of vertebral types os suggested by Williams (1939). 



notochord. The sclerotome, having a thickness of only one 

 or a few cells, is very restricted. The close relation of 

 muscle to notochord is explained by the fact that the larva 

 is soon to swim actively. 



Because the sclerotome is so restricted, intrasegmental 

 clefts cannot be detected. The first evidence of the skeleton 

 is the appearance of the procartilaginous neural arches in 

 the sheath of the neural canal. These arches extend up from 

 the notochord on either side of the neural tube. Their bases 

 lie in contact with a perichordal tube of cells. The peri- 

 chordal tube is thickest in the intervertebral areas, where it 



forms an intervertebral ring of circularly arranged fusiform 

 cells. This ring is thickest at the line of junction with the myo- 

 septum (Figure 6-19). The myoseptum extends up and back 

 along the neural arch and, in the caudal region, down and 

 back along the hemal arch. 



As development progresses, the intervertebral ring be- 

 comes cartilaginous and constricts the notochord (Figure 

 6-20). Ossification and chondrification appear at about the 

 same time in the thin vertebral rings of the perichordal 

 sheath. Externally the vertebral rings become bony, but 

 the inner cartilage is continuous with the interverte- 



154 



THE VERTEBRATE BODY SKELETON 



