[31] THE EVOLUTION OF THE FINS OF FISHES. 1011 



surfaces lie immediately against the epidermis, except at their bases, 

 where it is apparent that more or less mesoblast intervenes between the 

 ends of the proximal halves of the rays and the epiblast. Such is the 

 case at least with the pectoral rays which are first laid down as dense 

 homogeneous membrane, in the same way as the primary rudiments of 

 the cranial membrane bones. A cartilaginous nodule (median actino- 

 phore) is also included by the proximal ends of the dorsal, ventral, 

 anal, and pectoral rays, while the cartilaginous part in the caudal rays 

 is the margin of the expanded hypural cartilages, which betray in the 

 arrangement of the nuclei a tendency toward the separation of homolo- 

 gous nodules at certain nodal points near the ends of these pieces, 

 though they probably never become discrete in consequence of the re- 

 duction and specialization which the elements of the caudal tin have 

 suffered in the most specialized fishes. It is therefore evident that if 

 the tract from whence the interspinous elements are formed is meso- 

 blastic and cartilaginous, and at the same time continuous at one time 

 with the nodules at the bases of the rays, that those basal elements are 

 not truly of dermal origin. These facts, at least, indicate that the 

 whole ray is not of dermal origin ; or that only its lateral osseous halves 

 developed in membrane can ever have had such an origin in the Teleostei. 



The marginal bands of cartilage b Fig. 1, PL IV, at the ends of the hy- 

 l)ural i)rocesses, and which are indistinctly differentiated by nodal ag- 

 gregations of nuclei which resemble in appearance the arrangement of 

 these bodies in the basibranchial plate or in the hyomaudibular and 

 symplectic bar at a time when the segmentation or sundering of these 

 pieces into distinct elements is about to occur, is a striking fact, and 

 one which leads me to think that the hypural processes are not simple. 

 These nodal aggregations of nuclei are also found at the tips of the 

 neural and haemal spines of such Teleosts as do not have the dorsal 

 and anal confluent with the caudal, and I am inclined to regard these ter- 

 minal cartilages also as epiphysial elements, or rudimentary median actiu- 

 ophores which at one time were separate, but which have in consequence 

 of a i)rocess of degeneration been suppressed. These rudimentary ele- 

 ments of the spines anterior to the caudal may be regarded as repre- 

 senting the nearly suppressed basilar interspinous elements, while those 

 cartilaginous borders which we find to terminate the hypural jiieces 

 may be considered with some show of probability to represent the basal 

 nodules or basilar interspinous pieces of the other dev^eloped median fins, 

 for we find that the proximal ends of the caudal rays clasp this part of the 

 hypural processes somewhat after the fashion in which the proximal 

 ends of the dorsal and anal rays clasp the basal cartilaginous actino- 

 phores in the embryo. 



The method of development of the parts of the segments of the median 

 or unpaired system of fins is therefore very similar to that of the paired 

 fins, as displayed in the course of their outgrowth. The continuity of 

 the coraco-scapular cartilaginous plate with the basal cartilages of the 



