OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 29 



half-arch on each side a bar of cartilage, the rib. This rib, as in the 

 case of the ribs of osseous fishes, passes outward immediately between 

 the peritoneum and the lateral muscles. 



Anteriorly, at the third vertebra, the bases of the lower arch 

 stand at the sides of the notochord close to the bases of the upper 

 arch, but I do not think that they at any time coalesce. As a series 

 of sections are. run through toward the rear, the bases of the lower 

 arches descend to a lower level and a greater space intervenes between 

 them and the upper arches. 



It has already been noted that the bases of the lower arches, 

 especially along the anterior portion of the vertebral column, send 

 downward towards the aorta on each side a process of cartilage. 

 As may be seen from Fig. 9, the lower end of this thickens and 

 grows downward, so as to project a slight distance below the noto- 

 chord. These lower swellings of the cartilage are, as may be sur- 

 mised, the beginnings of the lowest cartilaginous rays which have 

 been described as occurring in the adult vertebrae, and which have 

 been called the aortal supports. My supposition, derived from the 

 examination of these structures in the adult, that they originate from 

 the lower arches is fully confirmed. As in the case of the lower 

 arches themselves these aortal supports develop from the front to the 

 rear. When a specimen 3omm. long is examined by means of cross- 

 sections, it is seen that, when about the twenty-fourth vertebra is 

 reached, the aortal support is cut off from the main cartilage by a 

 narrow spicule of bone. Behind this region the support is apparently 

 developed independently of the main mass of the lower arch. The 

 whole appearance suggests that this terminal portion of the arch- 

 base, becomes developed into true cartilage just a little too late to 

 escape being separated from its parent mass by the now developing 

 bone. 



Scheele (55) has observed in certain Cyprinidae (Rhodeus) struc- 

 tures which, consisting of fibrous tissue, arise from the lower and 

 hinder portions of the lower arches, and descending on each side 

 unite below the aorta. This vessel is therefore surrounded below 

 each vertebral body by this structure and supported and protected 

 by it. Scheele regarded it as an outgrowth from the base of the 

 lower arch, and I think that he was correct in his conclusion. The 

 structures which he describes are undoubtedly homologous with the 

 aortal supports of Amia. But in the Cyprinidae there has occurred 

 an enormous reduction of the cartilage which usually enters into the 

 formation of the axial structures, and these supports have felt the 

 effect of it. I think that in most fishes there will be found on the 



