/26 DEVELOPMENT OF THE SKELETON 



right angles from the posterior side of the pectoral or pelvic 

 girdle, and running parallel to the long axis of the body along 

 the base of the fin. The outer side of this bar is continued into 

 a thin plate, which extends into the fin. 



The structure of the skeleton of the fin slightly after its first 

 differentiation will be best understood from Plate 33, fig. T, and 

 Plate 33, fig. 7. These figures represent transverse sections 

 through the pelvic and pectoral fins of the same embryo on the 

 same scale. The basal bar is seen at bp, and the plate at this 

 stage (which is considerably later than the first differentiation) 

 already partially segmented into rays at br. Outside the region 

 of the cartilaginous plate is seen the fringe with the horny fibres 

 (h. f.) ; and dorsally and ventrally to the cartilaginous skeleton 

 are seen the already well-differentiated muscles (#2). 



The pectoral fin is shewn in horizontal section in Plate 33, 

 fig. 6, at a somewhat earlier stage than that to which the trans- 

 verse sections belong. The pectoral girdle (p. g^) is cut trans- 

 versely, and is seen to be perfectly continuous with the basal 

 bar (vp) of the fin. A similar continuity between the basal bar 

 of the pelvic fin and the pelvic girdle is shewn in Plate 33, fig. 2, 

 at a somewhat later stage. The plate continuous with the basal 

 bar of the fin is at first, to a considerable extent in the pectoral, 

 and to some extent in the pelvic fin, a continuous lamina, which 

 subsequently segments into rays. In the parts of the plate 

 which eventually form distinct rays, however, almost from the 

 first the cells are more concentrated than in those parts which 

 will form the tissue between the rays ; and I am not inclined to 

 lay any stress whatever upon the fact of the cartilaginous fin-rays 

 being primitively part of a continuous lamina, but regard it as a 

 secondary phenomenon, dependent on the mode of conversion of 

 embryonic mesoblast cells into cartilage. In all cases the sepa- 

 ration into distinct rays is to a large extent completed before 

 the tissue of which the plates are formed is sufficiently differ- 

 entiated to be called cartilage by an histologist. 



The general position of the fins in relation to the body, and 

 their relative sizes, may be gathered from Plate 33, figs. 4 and 5 

 which represent transverse sections of the same embryo as that 

 from which the transverse sections shewing the fin on a larger 

 scale were taken. 



