66 Traces of Unity in the 



in their nature to those of the oblique processes, and d( 

 the elements of the quasi-vertebral columns at the base 

 of the rays of the median fins in certain fishes. 



It may perhaps be an open question whether this 

 latter inference is fully justified by the facts: it can 

 scarcely be doubtful that a very different view must be 

 taken of the ' body ' of the vertebra to that which is 

 commonly taken a view in which this part is regarded, 

 not as a primary centre, but as a centre formed secon- 

 darily by the coalescence of two of the several centres 

 which make their appearance at different points of a 

 simple ring or zone a view which finds in the bony 

 part of the vertebra and in the nervous and vascular 

 parts, the same common plan, namely, a simple ring 

 which may become nodulated at certain points by the 

 development of centres which may be osseous or neural 

 or cardiac as the case may be, and which simple ring 

 may become divided by the intergrowth of centres on 

 opposite sides so as to form vertebral body or spinal 

 cord or heart as the case may be a view which, by 

 dividing the ' body ' in this way, opens out the vertebra 

 into a simple ring, and by so doing breaks down the 

 partition between it and the annellus. 



Nor is this view invalidated by anything that is 

 seen when it is looked into more particularly. 



The zones of the carapace of the tortoise and of the 

 plastron of the crab are forms in which the vertebra and 

 the annellus are brought very closely together. In the 

 chelonian the soft tissues are absent externally and the 

 conformation, in this respect, is substantially that which 

 is typical, not of the vertebra, but of the annellus : and 

 in the crustacean the calcareous processes which project 

 inwardly and form a grove for the lodgement of the 

 great central nervous system, and which in some 



