4 Mr. E. C. Punnett. On the Composition and Variations 



vertebrae presented somewhat more difficulty. Over each centrum in 

 the trunk, and towards its caudal end, issue the dorsal and ventral 

 roots of a spinal nerve the latter being rather more rostrally situated 

 than the former. As the nerve whose ventral root issues between the 



Fift. 1. Hinder portion of skull and first few vertebrae. Seen after a nearly 

 median sagittal section. 



skull and first vertebra has been taken as :, it follows that the first 

 spinal nerve will issue out over the first centrum, the second nerve 

 over the second centrum, and so on. This relation is always kept up 

 as we pass caudally along the vertebral column, i.e., a nerve of any 

 given serial number always leaves the column over the caudal end of 

 a centrum having the same serial number. When, however, we reach 

 a point on the vertebral column somewhere between the 45th and 50th 



Cenbrum48 



FlG. 2a. 



Gen C mm 43 

 FIG. 2b. 



Two vertebral columns at region where the transition from half- to whole- 

 vertebrae occurs. Seen from the inside after bisection and removal of the 

 dorsal cartilages of one side. 



