OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 31 



sented, and the cartilages that rest on them above and below. Wher- 

 ever these cartilages come in contact with the notochord, the latter 

 structure is seen to be slightly constricted, more in the case of the 

 larger cartilages than of the smaller ones. 



But the cartilaginous structures are of the greatest interest to us. 

 We are now in the region in which, in the adult, there occurs a change 

 from the single to the double vertebrae. The arches which are seen 

 on the extreme left of the figure are those of the second caudal ver- 

 tebra Between the long haemapophyses toward the right are seen 

 three small nodules of cartilage. On the upper side of the notochord 

 are seen other neural arches, and between the bases of those on the 

 right hand two small cartilages like those found between the lower 

 arches. But it is evident from the section that a third little mass of 

 cartilage has fused with the base of the next half-arch toward the 

 right. The section being taken a little to one side of the middle line, 

 it is evident that for each little nodule of cartilage seen, there is an- 

 other on the opposite side. Hence there would be four such masses 

 in front of the upper and lower arches belonging to each myomere. 



It will be further observed that the upper arches on the left side 

 of the figure appear to be segmented at short distance below their 

 bases, while the upper arches standing behind the small nodules of 

 cartilage are not thus segmented. This suggests that those basal 

 pieces on which the larger portion of each half-arch appears to rest 

 are really the equivalents of those little masses which lie between the 

 undivided arches. This is shown, too, I think, not only from the 

 fact that the little cartilages of the middle tail region and the basal 

 pieces of the region in front of them occupy corresponding positions, 

 but also because of the similarity in the manner of their development. 

 Both are evidently formed in the intervals between the bases of the 

 arches immediately in front and behind. Those of the dorsal and an- 

 terior tail region secondarily push themselves backward under the 

 bases of the neural arches. The upper arches become differentiated 

 first, and their bases come down to the membrana elastica. At the 

 same time incipient cartilage cells may be made out lying between 

 the bases, possibly continuous with them. Later, these also are trans- 

 formed into clear cartilage, and increasing in size so as almost to 

 touch one another, they push the bases of the neural arches away 

 from contact with the notochord. 



In the case of the lower arches of the dorsal region, we do not 

 find little cartilages lying between them. On the contrary, we find 

 that the anterior end of the base of each half-arch has grown forward, 

 vso as to occupy the place of the missing nodule. Or, it is posssible 



