THE TRANSVERSE PROCESSES OF NECTURUS 559 



the capitular head is borne upon an extension of the parapophysis, 

 while the tubercular head is attached to a process of the rib-bearer. 

 It is of interest to note that the higher position of the first three 

 ribs is in some way correlated with the formation of a direct 

 connection between the tubercular head of the rib and the dorsal 

 process of the rib-bearer. It is also important to note that the 

 relation of the rib to the rib-bearer in these anterior vertebrae is 

 almost exactly similar to the relation^ of the same structures 

 throughout the trunk region of Amblystoma and Salamandra. 

 The one minor difference between them is that in Necturus the 

 rib-bearer is generally separated from the cartilage of the neural 

 arch by connective tissue, while in Amblystoma and Salamandra 

 the two are continuous. As shown in figure 19, however, this 

 continuity often obtains in Necturus. 



DEVELOPMENT OF VERTEBRAE IN THE TAIL-TRUNK 

 TRANSITIONAL REGION 



In the trunk region of the younger larvae (21 to 22 mm.) it 

 has been seen that distinct ventral cartilages or haemapophyses 

 often appear. In some vertebrae these are independent elements 

 and in others are fused with the bases of the parapophyses and 

 appear as ventral projections of the same. In several cases the 

 basal stump appears to be growing in two directions, laterally and 

 ventrally, although the lateral element becomes dominant and 

 the ventral element in time disappears. 



Fig. 23 Transection through a second vertebra of a 70-mm. larva, c.c, 

 chordal cartilage; d.j)., dorsal process of rib-bearer; v.'p., ventral process of rib- 

 bearer; n.a., neural arch; t;. a., vertebral artery; r.h., rib-bearer; v.a., parapophysis. 



Fig. 24 More caudal transection of the same vertebra, d.p., dorsal process 

 of rib-bearer; ex., chordal cartilage; v.p., ventral process of rib-bearer; v.a., 

 vertebral artery; j).a., parapophysis. 



Fig. 25 Transection through the last trunk vertebra of a 25-mm. larva, n.a., 

 neural arch; r.h. p., rib-bearer proton; p.o., parapophysis", h.a., haemapophysis. 



Fig. 26 Transection through the first tail vertebra of a 25-mm. larva, n.a., 

 neural arch; r.h. 'p., rib-bearer proton; -p.a., parapophysis; h.a., haemapophysis. 



Fig. 27 Transection through the second tail vertebra of a 26-mm. larva. 

 i.v.r., intervertebral ring; n.c, notochord; h.a., haemapophysis; p. a., para- 

 pophysis; n.a., neural arch. 



