[37] THE EVOLUTION OF THE FINS OF Fir HES. 1017 



part of the fold, and it would thns seem tluit the ombryoiiic rays were 

 formed from within outwards or in the direction of the migTation of the 

 tissue from which they are formed, which seems to be converted into 

 the thin, nearly homogeneous membrane already spoken of, before these 

 rays become distinctly diiferentiated as fibers which present a remark- 

 able homogeneity of structure. Fig. 5, PI. IX, shows liow deeply the 

 membranous rudiments of the permanent rays may become imbedded 

 in the mesoblast at their proximal extremities. 



The development of the embryonic rays between the epidermis and 

 mesoblast of the folds which give rise to the paired fins is precisely 

 similar to that which occurs in the folds from which the unpaired fins 

 are developed, as may be g'athered from a glance at Figs. 3 and 5 on 

 Plate X. 



VII. — What is it that constitutes a fin-ray? 



As may be gathered from what has preceded u])oii the origin and 

 development of the fin-rays, it becomes pertinent to inquire what it is 

 that constitutes a true fin-ray. It may be said that true fin-rays, which 

 are embryologically the bomologues of each other, are formed only in 

 the group distinguished under the name of Lvrifera by the most recent 

 taxouomists, a group embracing- the SelachU, HoJocephall, CJwndrostei, 

 Holosfei (and Ganoidei generally). Dipnoi, Physostomi, atul Fhysoclysti. 

 It is also a fact that the development of the fii st traces of the rays is 

 approximately the same ; that is, they are first formed just under the 

 larval integument or epiblast (the epidermis of the adult) in all of these 

 forms. In no case are they certainly known to develop in cartilage. 

 The apparent exceptions to this statement, such as the cartilaginous 

 radii found in the tins of the Rays, it must be borne in mind are in no 

 sense the homologues of the horny rays of a form even so closely allied 

 to them as the Sharks, because these radii of the pectorals, for example, 

 in the Rays, are the exact homologues of the cartilage cut across in the 

 Salmon's pectoral, hp in Fig. 5, PI. X, or that of the pectoral. Fig. 3, tr, 

 PI. X, of ScylUum. The cartilaginous rays or actinopliores of the Lyri- 

 Jera develop in the central, axial, or medullary mesoblast of the fin folds 

 in both the paired and unpaired fins. These elements may be segmented 

 into basilar iuterneural, interneural, basilar interhsemal, and interhaemal 

 elements in the paired fins, or even be partially or wholly wanting 

 where the morphological differentiation of the fin has not advanced 

 much beyond an embryonic condition, as in the case of the adipose fins, 

 for example ; Or they may be more or less suppressed by other causes, 

 such as degeneration in the caudal region, and fused together, as else- 

 where stated. In Folypterus the dorsal finlets show evidence of con- 

 crescence and a tendency towards the formation of a uniserial structure 

 with a loss of the basal elements of the coucresced rays. In the paired 

 fins oi Dipnoi these axial cartilages maybe reduced to a single tapering 

 multisegmented bar, as in Protopterus, with a few short lateral elements 



