542 D. L. GAMBLE 



of the basal stump appears. When it does form it has a dis- 

 tinctly different appearance; that is, it is weaker and made up 

 of fewer cells and stains much less intensely with haematoxylin. 



This difference between the proton of the rib and rib-bearer on 

 one hand and that of the basal stump on the other, points to the 

 former as elements formed in the muscle septa and seems to 

 indicate that the basal stump only is a lateral product of a verte- 

 bral element. 



In a slightly more advanced larva wliich also measured 20 mm. 

 the basal stump makes its first appearance (fig. 3, y.a.). It is 

 seen here as a cartilaginous process projecting laterally from the 

 notochord. This cartilage in the fourth trunk vertebra does not 

 lie in a horizontal plane, but projects slightly dorsad as well as 

 laterad. 



At its tip the hyaline matrix disappears and it becomes con- 

 tinuous with a strand of procartilage cells (the proton of the rib- 

 bearer) which extends upward to the neural arch. At a level 

 with the base of the neural arch this strand of cells is also con- 

 tinuous with the rib proton (fig. 3, r.p.) which passes into the 

 horizontal septum. Farther back in the trunk region the prox- 

 imal end of the rib proton appears on a level with the middle 

 of the notochord and here the basal stump Ues horizontally 

 (fig. 4, p.a.) and is directly continuous with the rib proton (fig. 4, 

 r.p.). When this is the case the distal end of the basal stump is 

 connected with the neural arch by a strand of procartilage cells 

 as in the anterior region (fig. 4, r.h.p.). 



In the 21-mm, larva the neural arch is completed dorsally, 

 but the basal stump appears no further advanced than in the 

 20-mm. larva. The first rib chondrification is to be noticed in 

 the anterior trunk region of the 21-mm. larva. Thi's first cartilage 

 is developed distally, and between it and the cartilage of the 

 basal stump procartilage is found. 



On the ventral side of the notochord just ventrad of the base 

 of the basal stump are found in many of the trunk vertebrae 

 small knobs of cartilage (fig. 4, h.a.). Occasionally two are found 

 in a single vertebra, one lying on either side of the aorta. Gen- 

 erally, however, only one is present. The hyaline matrix of 



