56 MR. F. M. BALFOUR ON THE SKELETON 



anatomist to elucidate the nature of the fins ; and his views 

 on this subject have undergone considerable changes in the 

 course of his investigations. After Giinther had worked out 

 the structure of the fin of Ceratodus, Gegenbaur suggested that 

 it constituted the most ^rixmiiwe persisting type of fin, and 

 haSj moreover, formed a theory as to the origin of the fins 

 founded on this view, to the efi'ect that the fins, together 

 with their respective girdles, are to be derived from visceral 

 arches with their rays. 



His views on this subject are clearly explained in the 

 subjoined passages quoted from the English translation of 

 his * Elements of Comparative Anatomy,' pp. 473 and 477. 



" The skeleton of the free appendage is attached to the extre- 

 mity of the girdle. When simplest, this is made up of cartilagi- 

 nous rods (rays), which differ in their size, segmentation, and 

 relation to one another. Otie of these rays is larger than 

 the rest, and has a number of other rays attached to its sides. 

 I have given the name of archipterygium to the ground-form 

 of the skeleton which extends from the limb-bearing girdle 

 into the free appendage. The primary ray is the stem of 

 this archipterygium, the characters of which enable us to 

 follow out the lines of development of the skeleton of the 

 appendage. Cartilaginous arches beset with the rays form 

 the branchial skeleton. The form of skeleton of the append- 

 ages may be compared with them ; and we are led to the 

 conclusion that it is possible that they may have been 

 derived from such forms. In the branchial skeleton of the 

 Selachii the cartilaginous bars are beset with simple rays. 

 In many a median one is developed to a greater size. As 

 the surrounding rays become smaller, and approach the 

 larger one we get an intermediate step towards that arrange- 

 ment in which the larger median ray carries a few smaller 

 ones, This differentiation of one ray, which is thereby 

 raised to a higher grade, may be connected with the primitive 

 form of the appendicular skeleton ; and as we compare the 

 girdle with a branchial arch, so we may compare the median 

 ray and its secondary investment of rays with the skeleton 

 of the free appendage. 



^'AU the varied forms which the skeleton of the free 

 appendages exhibits may be derived from a ground-form 

 which persists in a few cases only, and which represents the 

 first, and consequently the lowest, stage of the skeleton in 

 the fin — the archipterygium. This is made up of a stem 

 which consists of jointed pieces of cartilage, which is articu- 

 lated to the shoulder-girdle and is beset on either side with 

 irays which are likewise jointed. In addition to the rays of 



