OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 33. 



either lifted tip out of contact with the notochord or be suppressed. If 

 in addition to this, the lower intercalated cartilages were to be sup- 

 pressed or to become fused with the bases of the lower arches, we 

 should have exactly the same condition that we find in the greater 

 part of the vertebral axis of the young Amia. 



' It has already been shown that in Amia the notochord is not at 

 any stage surrounded by a tube of cartilage. At most there are to 

 be found only four bands of more or less differentiated cartilage, two 

 above and two below the notochord, and it is not certain that these 

 bands are continuous. In Lcpisosteus, however, according to Balfour 

 and Parker (4) the notochord is at one stage almost completely en- 

 closed by a tube of well defined cartilage. The tube is interrupted 

 only by gaps at the sides of the notochord between the bases of the 

 upper and the lower arches. Nothing like distinct and independent 

 intercalated cartilages were observed" by these authors; but "inter- 

 vertebral rings of cartilage " were observed and figured by them. 

 These were placed in the intervals between successive pairs of arches, 

 and completely surround the section of the notochord where they 

 occur. Balfour and Parker found that these rings do not have a uni- 

 form thickness, but that each showed two dorsal and two ventral 

 thickenings. These thickenings are in line with the bases of the 

 arches, and were regarded by the observers as presumptive evidence 

 that the rings had been produced through the fusion of the bases. 

 However, there were missing the very stages in which the rings took 

 their origin, and it is by no means certain that they are not produced 

 from the fusion of four intercalated cartilages for each body segment. 

 If the four intercalated cartilages of each segment in the tail of Amia 

 should expand and then fuse with one another and with the adjacent 

 bases of arches, we should have the condition found in Lepisosteus. 



Ossification of the vertebral column begins probably soon after 

 the young Amia has reached a length of i5mm. At least, no deposit 

 of bony matter has been observed in specimens of this length, while 

 such deposits are abundant in a specimen 23mm. long. 



The place of origin of the earliest formed bone of the vertebra? 

 of fishes has been much discussed and has received from investiga- 

 tors the most varied and often contradictory answers. In the settle- 

 ment of this question is further involved the view that is to be held 

 concerning the composition of the vertebra, especially whether or 

 not any part of it is derived from the envelops of the notochord. In 

 discussing this subject we shall, for the sake of brevity, consider that 

 any part of the notochord or of its sheaths which does not change to 

 either cartilage or bone does not form a portion of the vertebra. 



