THE TRANSVERSE PROCESSES OF NECTURUS 539 



of the rib-bearer in Necturus, is much shortened and is indis- 

 tinguishable from the rib-bearer. Now, the rib is attached 

 directly to the neural arch, although it has never actually lost its 

 connection with the distal end of the basal stump. According 

 to this interpretation of the dorsal shifting of the rib attachment, 

 the rib-bearer is substituted for the proximal portion of the basal 

 stump in attaching the rib to the vertebral column. 



In view of these conflicting opinions regarding the independence 

 of the rib and the way in which its attachment changes, and 

 because of the possibility that the two stages (24 and 43 mm.) 

 studied by Goeppert failed to furnish all the facts, it was deemed 

 advisable to undertake a study of the development of the ribs 

 and transverse processes in a more complete series. Accord- 

 ingly, Necturus larvae of the following stages were studied: 

 20, 21, 22, 23, 24, 25, 26, 30, 40, 50, and 70 mm., respectively, 

 and in addition various stages of other species. 



After a study of these stages, it is believed that the dorsal 

 shifting of the ribs in urodeles is not the comphcated process that 

 Goeppert supposed and that all the steps in this change in attach- 

 ment can be illustrated in the single form, Necturus. 



In discussing the development of these structures the follow- 

 ing order will be followed: 1) Fourth to antepenultimate trunk 

 vertebrae. 2) First three vertebrae. 3) Definitive estate of trunk 

 vertebrae. 4) Last trunk and first and second tail vertebrae. 



DEVELOPMENT OF TRUNK VERTEBRAE (FOURTH TO 

 ANTEPENULTIMATE) 



In the 20-mm. larva (fig. 2) the only cartilage present is that 

 of the neural arch which is not yet complete dorsally. The 

 neural arches of consecutive vertebrae are separated by quite 

 an interval. The sheath of the notochord is made up of an outer 

 skeletogenous layer, the elastica externa, within which is a thinner 

 layer, the elastica interna, while the outer cells of the notochord 

 are arranged in an epitheUal-hke layer, the notochordal epithe- 

 lium. In the middle of the future centrum the notochord sheath 

 is thin and all three layers are close together. Intervertebrally 

 the sheath is much thicker, due to the great increase in the num- 



