THE TRANSVERSE PROCESSES OF XECTURUS 549 



it is believed by the writer that all of the steps in this shifting of 

 attachment can be illustrated by this one form alone. In 

 Necturus the horizontal septum is relatively high in the extreme 

 anterior region of the trunk and is lower posteriorly, and there- 

 fore might be looked to with a reasonable amount of assurance, 

 to show how the rib has become attached to the neural arch. 



This has already been suggested by Wilder ('03) in his memoir 

 on the skeletal system of Necturus. Wilder makes the following 

 statement in this connection: ''It would thus seem, judging from 

 the purely anatomical evidence, that the condition described by 

 Goeppert as characteristic of Necturus, is not a universal one 

 applicable to all the vertebrae, but is restricted to a certain 

 region approximately that of vertebrae 8 to 18. " 



"It would seem important to investigate the development of 

 the transverse process and rib in certain of the other vertebrae, 

 for example the second and the fourth. " 



It was found in the present study that Wilder's suggestion 

 was a good one and that the second, third, and fourth vertebrae 

 are important in making intelligible the morphology of these 

 structures. 



The first vertebra never bears a definitive rib, although a 

 cartilaginous rib rudiment was found in this vertebra in a 23-mm. 

 larva (fig. 14). The first vertebra has one peculiarity, however, 

 which distinguishes it from all the other vertebrae. The bases 

 of the neural arch of the first vertebra are enlarged and extend 

 much farther ventrad than in others. These enlargements seem 

 to represent basal stumps which have fused with the bases of the 

 neural arch (fig. 11). Continuity of neural arch and basal 

 stump was seen in none of the other vertebrae with a single excep- 

 tion (fig. 12). In a 21-mm. larva in the second vertebra the 

 neural arch extended ventrally exactly as in the first. In follow- 

 ing back through the sections, however, it was seen that the 

 ventrally projecting end of the neural arch extended caudad so 

 that a section which passed through the vertebral column just 

 caudad of the neural arch would cut through these caudal pro- 

 jections. Here, in cross-section, they appear as typical basal 

 stumps before the appearance of any lateral cartilage (fig. 13, h.s.). 



