THE ENDOSKELETON 



125 



The next few stages in the history of these parts, lying 

 between the condition above described and definite vertebrae, 

 are still somewhat a matter of controversy, since, in the various 



a 



c 



d 



FIG. 33. Diagrams illustrating a theory of the development of the ver- 

 tebrate. 



(a) Condition previous to the formation of vertebral anlagen (caudal region). 

 The body is divided into segments by transverse myocommata through which run the 

 notochord, the nerve cord, and the aorta. In the region of the coelom the myocom- 

 mata open ventrally and allow the alimentary canal to pass. There is no trace 

 of bone or cartilage, (b) Later stage, in which skeletal bridges have formed along 

 the edges of the myocommata, both dorsally and ventrally. (c) Detail of stiil later 

 stage, in which the sheath of the notochord has chrondrified (or ossified) at the 

 points where the bridges come in contact with it. (d) Completed vertebrae, formed 

 by the fusion of the elements shown in (c). 



types of fish, where these stages should be sought, numerous 

 modifications have taken place which are to be explained as 



