THE TRANSVERSE PROCESSES OF NECTURUS 553 



surrounding cartilage (fig. 15, h.a.). Cartilage has developed 

 in the inner edge of the transverse septum along its Hne of attach- 

 ment to the neural arch, and this extends downward and inward 

 toM^ard the upwardly developing parapophysis, from which it is 

 separated by mesenchymal cells (fig. 16, r.h.). 



In the next (third) vertebra, cartilage has not formed all along 

 the line of attachment of the transverse septum to the neural 

 arch, but is found only at its ventral end (fig. 17, r.h.). In other 

 words, the rib-bearer appears as a knob of cartilage projecting 

 ventrolaterally from the neural arch, while the parapophysis 

 extends dorsolaterally toward it, the two being separated by a 

 considerable interval. The proton of the rib is continuous with 

 the cartilage attached to the neural arch (rib-bearer) and has 

 no connection with the lateral cartilage (parapophj^sis) (fig. 17). 



It will have been noted by this time that the second and third 

 vertebrae differ in several ways from those farther back in the 

 trunk. These differences can be summed up as follows : 



1. Developmental processes seem to be retarded in this 

 region. The parapophyses and the rib-bearer unite much later, 

 the cartilage of the rib develops mesally more slowly, and the 

 haemapophyses persist longer than in the mid trunk region. 

 As before mentioned, this relative slowing up of the development 

 of these structures may be an expression of the reduction of ribs 

 in the cervical region of higher vertebrates. 



2. The parapophyses do not lie in a horizontal plane, but 

 project dorsolaterally. 



Fig. 17 Transection through the third vertebra of a 22-mm. larva, n.a., 

 neural arch; r.b., rib-bearer; p.a., parapophysis. 



Fig. 18 Transection through the second vertebra of a 24-mm. larva, n.a., 

 neural arch; r.b., rib-bearer; p.a., parapophysis; h.a., haemapophysis; b.s., 

 basal stump. 



Fig. 19 Transection through the third vertebra of a 24-mm. larva, n.c, 

 notochord; n.a., neural arch; r.b., rib-bearer; p.a., parapophysis. 



Fig. 20 Transection through the fourth vertebra of a 24-mm. larva, n.a., 

 neural arch; r.b., rib-bearer; r., rib; p.a., parapophysis. 



Fig. 21 Transection through the second vertebra of a 25-mm. larva, n.a., 

 neural arch; r.b., rib-bearer; r., rib; p.a., parapophysis. 



Fig. 22 Transection through a trunk vertebra of Amblystoma. n.a., neural 

 arch; r., rib; r.b., rib-bearer. 



