666 MR. F. M. BALFOUR ON THE SKELETON [JuDB 7, 



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 LVII. figs. 4 and 5, 

 which represent transverse sections of the same embryo as that from 

 which the transverse sections showing the fin on a larger scale were 

 taken. 



During the first stage of its development the skeleton of both 

 fins may thus be described as consisting of a longitudinal bar running 

 along the base of the Jin, 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 continuous in front with the pectoral or 

 pelvic girdle, as the case may be. 



The further development of the primitive skeleton is different in 

 the case of the two fins. 



The Pelvic Fin. — The changes in the pelvic fin are comparatively 

 slight. Plate LVII. fig. 2 is a representation of the fin and its skeleton 

 in a female of Scyllium stellare shortly after the primitive tissue is 

 converted into cartilage, but while it is still so soft as to require the 

 very greatest care in dissection. The fin itself forms a simple pro- 

 jection of the side of the body. The skeleton consists of a basi- 

 pterygium {bp), continuous in front with the pelvic girdle. To the 

 outer side of the basipterygium a series of cartilaginous fin-rays are 

 attached — the posterior ray forming a direct prolongation of the 

 basipterygium, while the anterior ray is united rather with the pelvic 

 girdle than with the basipterygium. All the cartilaginous fin-rays 

 except the first are completely continuous with the basipterygium, 

 their structure in section being hardly different from that shown in 

 Plate LVII. fig. 1. 



The external form of the fin does not change very greatly in the 

 course of the further development ; but the hinder part of the attached 

 border is, to some extent, separated off from the wall of the body, 

 and becomes the posterior border of the adult fin. With the 

 exception of a certain amount of segmentation in the rays, the 

 character of the skeleton remains almost as in the embryo. The 

 changes which take place are illustrated by Plate LVII. fig. 3, showing 

 the fin of a young male of Scyllium stellare. The basipterygium 

 has become somewhat thicker, but is still continuous in front with 

 the pelvic girdle, and otherwise retains its earher characters. The 

 cartilaginous fin-rays have now become segmented off from it and 

 from the pelvic girdle, the posterior end of the basipterygial bar 

 being segmented off as the terminal ray. 



The anterior ray is directly articulated with the pelvic girdle, and 

 the remaining rays continue articulated with the basipterygium. 

 Some of the latter are partially segmented. 



As may be gathered by comparing the figure of the fin at the 

 stage just described with that of the adult fin (woodcut, fig. 2), the 

 remaining changes are very slight. The most important is the 

 segmentation of the basipterygial bar from the pelvic girdle. 



