THE TRANSVERSE PROCESSES OF XECTURUS 551 



Extending downward and inward from the neural arch is the 

 strand of procartilage cells in which wdll later form the cartilage 

 of the rib-bearer. It is very noticeable here that this condensa- 

 tion of cells weakens markedly and finally disappears in passing 

 toward the notochord. It is further seen that a mesenchymal 

 condensation has appeared all along the inner edge of the trans- 

 verse septum where it attaches to the neural arch. This 

 represents the dorsal continuation of the rib-bearer proton. 

 Cartilage appears at this stage in the extreme distal portion of 

 the rib. 



In the third vertebra a broad cartilage appears on the ventral 

 side of the notochord on one side only, while on neither side is 

 there any suggestion of a lateral cartilage. This cartilage is to 

 be interpreted as basal stump before the development of the 

 lateral cartilage (parapophysis) . As in the preceding vertebra, 

 the procartilage cells are seen in the inner edge of the transverse 

 septum, where it is attached to the neural arch and also between 

 the neural arch and notochord. Here again is noticed the decided 

 weakening of this proton mesally. The beginning of the rib pro- 

 ton is on a level with the base of the neural arch. 



In the second vertebra of a 22-mm. larva (fig. 15) the basal 

 stump is very broad at the base. The direction of growth is 

 dorsolateral (p.a.), this process dominating the development of 

 the ventral projection of the basal stump (haemapophysis) which 

 appears as a single cartilage cell much more advanced than the 



Fig. 11 Transection through the first vertebra of a 20-mm. larva, b.s., 

 basal stump; n. a., neural arch. 



Fig. 12 Transection through the anterior part of the second vertebra of a 

 21-mm. larva, n.a., neural arch; b.s., basal stump. 



Fig. 13 Transection through the posterior part of same vertebra, n.a., 

 neural arch; b.s., basal stump. 



Fig. 14 Transection through the vertebral column between the first and 

 second vertebrae, e.m., epaxial muscle mass; e.i., elastica interna; h.m., hy- 

 paxial muscle mass;z.r., intervertebral ring; l.l.a., lateral-line artery; 1. 1. n , lat- 

 eral-line nerve; n.c, notochord; r., rib; r.a., right aorta. 



Fig. 15 Transection through the second vertebra of a 22-mm. larva, n.a., 

 neural arch; p.a., parapophysis; /;. a., haemapophj-sis; b.s., basal stump. 



Fig. 16 A more caudal transection of the same vertebra, n.a., neural arch; 

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



