THE TRANSVERSE PROCESSES OF NECTURUS 545 



The ventral cartilages were first seen in a 20-mm. Necturus 

 larva (fig, 6, h.a.). Here a lateral outgrowth of the ventral 

 cartilages is just beginning to appear (p.a.). Although these 

 two processes are continuous, it is very evident that the cartilage 

 of the ventral element is better developed and that the cells of 

 the lateral process have just begun to secrete a hyaline matrix. 

 This shows clearly that the primitive basal stump is growing in 

 two directions, ventrally giving rise to a haemapophysis and 

 laterally becoming the parapophysis. When bone forms in the 

 skeletogenous layer of the notochord the haemapophyses of the 

 trunk disappear. From this point on the terms of Owen will be 

 used while discussing these 'derivatives' of the basal stump. 

 The lateral cartilage will be called the parapophysis and the 

 ventral, the haemapophysis. 



In the 22-mm. larva the cartilage of the rib has developed 

 mesally so that the area of procartilage between it and the para- 

 pophysis is much less extensive. Caudad of the third vertebra 

 the ventral cartilages or haemapophyses do not appear, the 

 reason for this being apparently that bone is beginning to make 

 its appearance in the outer layer of the notochordal sheath. 

 In this stage (fig. 7) in the fourth and fifth vertebrae the para- 

 pophysis makes a cartilaginous connection with the rib-bearer 

 for the first time. Because of the fact that in these vertebrae the 

 basal stump tends to project dorsolaterally, no distinction can 

 be made between it and the rib-bearer. In passing caudally, 

 however, it will be seen that as the parapophysis comes to he in 

 the horizontal plane, the cartilage of the rib-bearer disappears 

 and in its place appears a strand of procartilage cells (fig. 8). 



In the 24-mm. larva structures are considerably more advanced. 

 The dorsal portion of the neural arch has begun to develop 

 caudally into a median posterior articular process and cephalad 

 into two anterior articular processes, one on either side of the 

 middorsal line. The parapophysis in the midtrunk region 

 projects laterally and in the strand of cells which connects it 

 and the neural arch in the preceding stage, cartilage appears 

 (fig. 9, r.6.). 



