720 DEVELOPMENT OF THE SKELETON 



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. 



"All the varied forms which the skeleton of the free ap- 

 pendages 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 articulated to the shoulder- 

 girdle and is beset on either side with rays which are likewise 

 jointed. In addition to the rays of the stem there are others 

 which are directly attached to the limb-girdle. 



" Ceratodus has a fin-skeleton of this form ; in it there is a 

 stem beset with two rows of rays. But there are no rays in the 

 shoulder-girdle. This biserial investment of rays on the stem 

 of the fin may also undergo various kinds of modifications. 

 Among the Dipnoi, Protopterus retains the medial row of rays 

 only, which have the form of fine rods of cartilage; in the 

 Selachii, on the other hand, the lateral rays are considerably 

 developed. The remains of the medial row are ordinarily quite 

 small, but they are always sufficiently distinct to justify us in 

 supposing that in higher forms the two sets of rays might be 

 better developed. Rays are still attached to the stem and are 

 connected with the shoulder-girdle by means of larger plates. 

 The joints of the rays are sometimes broken up into polygonal 

 plates which may further fuse with one another ; concrescence of 

 this kind may also affect the pieces which form the base of the 

 fin. By regarding the free rays, which are attached to these 

 basal pieces, as belonging to these basal portions, we are able to 



