OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 27 



the cartilage of the mass is in the undifferentiated condition and 

 when it has become quite hyaline, a section may be found in which 

 the bases and the intermediate masses appear to be quite continuous. 

 These masses are, as is to be expected, best developed in front, and 

 become, in this stage, very obscure about the middle of the back. 

 They rest immediately on the elastica, while the bases of the arches- 

 seem to have been crowded upward out of contact with it. If these 

 little cartilages just described are ever actually continuous with the 

 bases of the arches immediately before and behind them, they cer- 

 tainly become first cut off from the bases just in front, and later from 

 the bases immediately behind them. 



In the region of the tail, masses of cells suggesting the early 

 stage of cartilage are seen to lie between the bases of the successive 

 half-arches, and often they cannot be sharply separated from the ad- 

 joining arch bases. This applies both to the upper and the lower sides. 

 of the notochord. While we may be somewhat uncertain whether 

 or not these newly appearing masses of cartilage have a direct con- 

 nection at any time with the bases of the arches, this much is very 

 certain : there is never at any stage a contimiation of the cartilage 

 of the bases, or of the intermediate masses of one side, across the noto- 

 chord to those of the other side. Nor have I found anywhere that 

 the cartilage of the upper arches is continuous with that of the lower 

 arches. Hence there is nothing resembling a tube of cartilage en~ 

 closing the notochord. 



In the anterior portion of the body the cartilages which repre- 

 sent the lower arches have not changed greatly from their earlier 

 condition. The cartilage is better differentiated, showing now cejls 

 lying in a distinct ground-mass. The masses appear in the form of 

 flat bands lying against the outer sheath of the notochord. The 

 bands come down quite close to the aorta, and it is at their lower end 

 that we find the cartilage best defined. In the hinder dorsal region 

 the bands appear to descend to a lower level on the sides of the noto- 

 chord, the lower ends coming into quite close contact with the aorta. 

 Furthermore, each band, or arch-base, sends outward from near its 

 upper end a short process into the region where later the proximal 

 end of a rib will appear. This may be seen throughout the whole 

 trunk region. In the tail the halves of each lower arch have united 

 at their distal ends, so as to enclose the blood vessels. 



My specimen next in length to those just described is 23mm. 

 long. In this the notochord has attained a diameter of about .4mm, 

 exclusive of the two sheaths. The epithelial layer has no longer the 

 distinct appearance that it had in the preceding stage. Here and 



