NECTURUS MAOULATUS. 



393 



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Fig. 4. 16th vertebra ; posterior end view. X 3. 



one, the pleurapophysis, is "associated 

 with the centrum. The investing bony 

 mass is formed secondarily about these 

 rods by periostosis and consists essen- 

 tially of two simple investing sheaths 

 and two flattened laminae, of which 

 one is vertical and dorsal, connecting 

 the two sheaths, and the other horizon- 

 tal and ventral, applied to the ventral 

 side of the pleurapophysial sheath and 

 spreading out proximally over nearly 



the entire length of the centrum, with wliich it becomes fused. This latter lamina is 

 perforated by the ventral foramen, situated immediately posterior to the pleurapophysis 

 and transmitting the collateral vertebral vessels. As the dorsal lamina meets the ventral 

 one nearly at right angles, they form with the side of the centrum, two fossae, the anterior 

 and the posterior vertebral fossae. The posterior fossa is much the deeper and communi- 

 cates with the region ventral to the vertebral column by means of the ventral foramen 

 just mentioned. 



In the larva the cartilaginous diapophysis and pleurapophysis are directly contin- 

 uous proximally with a lateral mass, the "rib bearer" (Rippentrager) of Goppert, while 

 distally they unite to form the cartilaginous rib, the whole mass being at this time a con- 

 tinuous piece. The process of ossification within and around the rib bearer and the sides 

 of the vertebrae, and especially the growth of the osseous sheaths about these rods, gradu- 

 ally' cut them off from one another and restrict them proximally into tapering points, so 

 that in a macerated adult vertebra, from which all the cartilage has been removed, the 

 moulds of the parts in question are seen as very deep and conical pockets. 



Later ossification separates the free rib from the encasing sheaths of the transverse 

 process, and joints are thus formed between the two bodies. The part of the rib thus 

 segmented from the diapo hysis forms the tubercular process and the part once in con- 

 nection with the pleurapophysis becomes the capitular process, both processes retaining 

 throughout life an articular connection with the rods from which they originally separated. 



Embryological investigation has rendered it probable that the pleurapophysis and 

 its associated capitular head represent the primary condition, and that the cartilage of the 

 rib bearer becomes dorsally extended for increase of support, developing later the second- 

 ary or tubercular attachment. Goppert has shown this secondary growth of the tuber- 

 cular connection in the case of Triton and some other Urodeles, a growth which proceeds 

 from the ribs and follows alono- a line of connective tissue until it reaches the rib bearer 



