62 MK.. F. M. BALFOUR ON THE SKELETON 



toral 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 difiPerentiation will be best understood from Plate IV, 

 fig. 1, and Plate V, fig. 7. These figures represent trans- 

 verse sections through the pelvic and pectoral fins of the 

 same embryo on the same scale. The basal bar is seen at 

 hp, and the plate at this stage (which is considerably later 

 than the first differentiation) already partially segmented 

 into rays at hr. Outside the region of the cartilaginous 

 plate is seen the fringe with the horny fibres (^./) ; and dor- 

 sally and ventrally to the cartilaginous skeleton are seen the 

 already well-diff'erentiated muscles (m). 



The pectoral fin is shown in horizontal section in Plate 

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

 transverse sections belong. The pectoral girdle {jp-g) is cut 

 transversely, and is seen to be perfectly continuous with the 

 basal bar {hp) of the fin. A similar continuity between the 

 basal bar of the pelvic fin and the pelvic girdle is shown in 

 Plate IV, fig. 2, at a somewhat later stage. The plate con- 

 tinuous with the basal bar of the fin is at first, to a consider- 

 able 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 dis- 

 tinct 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 

 separation into distinct rays is to a large extent completed 

 before the tissue of which the plates are formed is sufficiently 

 differentiated 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 IV, figs. 

 4 and 5, which represent transverse sections of the same 

 embryo as that from which the tranverse sections showing 

 the fin on the larger scale were taken. 



During the first stage of its development the skeleton of 

 both fins may thus be described as consisting of a longi- 

 tudinal bar running along the base of the fin, and giving off at 

 right angles series of rays which pass into the fin. The 

 longitudinal bar may be called the basipterygium ; and it is 



