in. I DEVELOPMENT. 19 



the same vertebra may be developed exogenously in one 

 animal and autogenously in another. 



In nearly all the more prominent processes, moreover, 

 whether formed by exogenous or autogenous ossification, 

 the extreme tip remains cartilaginous for a considerable 

 time ; and at a comparatively late period in developmental 

 life (near the approach of maturity) a small ossific centre 

 forms in it. This spreads through the cartilage, and then 

 constitutes an epiphysis, which ultimately unites to, and 

 becomes indistinguishably incorporated with, the remainder 

 of the process. 1 



The spinous process is either formed by the coalescence 

 of outgrowths from the two pieces forming the neural arch, 

 or the greater part of it may be (as in the long spines of the 

 anterior thoracic vertebras of Ungulates) formed by a very 

 early autogenous ossification, which soon becomes united to 

 the upper part of the arch. In either case it is usually com 

 pleted by an epiphysis of comparatively late ossification. 



There is one part connected with certain vertebrae which 

 requires some particular consideration, on account of the 

 great modifications it presents, being in some regions a 

 largely developed independent bone, articulated with the 

 vertebra by synovial joints, and in. other regions a small 

 rudiment, early and firmly united to, and incorporated with, 

 the vertebra itself. 



The ribs in the thoracic region, though primarily formed 

 from a rod of cartilage continuous with that of the vertebra, 



1 These epiphyses are sources of considerable difficulty in tracing 

 homologous parts, as it is questionable whether they should be treated 

 as separate elements of the skeleton, and, if not, where to draw the 

 line between an epiphysis and an element. They often appear mere 

 conveniences of growth, as it were, being developed upon the end of 

 a process when it is long, and being absent in a corresponding part of 

 stunted dimensions. 



C 2 



