1881.] OF THE PAIRED FINS OF ELASMOBRANCHS. 665 



two fins of each side is nearly, though not quite, longitudinal, 

 sloping somewhat obliquely ventralwards. It thus comes about that 

 the attachment of each pair of limbs is somewhat on a slant, and 

 that the pelvic pair nearly meet each other in the median ventral 

 line shortly behind the anus. 



The embryonic muscle-plates, as I have elsewhere shown, grow 

 into the bases of the fins ; and the cells derived from these ingrowths, 

 which are placed on the dorsal and ventral surfaces in immediate 

 contact with the epiblast, probably give rise to the dorsal and ventral 

 muscular layers of the limb, which are shown in section in 

 Plate LVII. fig. 1 m and in Plate LVIII. fig. 7 m. 



_ The cartilaginous skeleton of the limbs is developed in the in- 

 different mesoblast cell between the two layers of muscles. Its early 

 development in both the pectoral and the pelvic fins is very similar. 

 When first visible it differs histologically from the adjacent mesoblast 

 simply in the fact of its cells being more concentrated ; while its 

 boundary is not sharply marked. 



_At this stage it can only be studied by means of sections. It 

 arises simultaneously and continuously with the pectoral and pelvic 

 girdles, and consists, in both fins, of a bar springing at right angles 

 from the posterior side of the pectoral or pelvic girdle, and runnino- 

 parallel to the long axis of the body along the base of the fin. The 

 outer side of this bar is continued into a thin plate, which extends 

 into the fin. 



The structure of the skeleton of the fin slightly after its first diffe- 

 rentiation will be best understood from Plate LVII. fig. 1, and Plate 

 LVIII. fig. 7. These figures represent transverse sections through 

 the pelvic and pectoral fins of the same embryo on the same scale. 

 The basal bar is seen at l/p, and the plate at this stage (which is 

 considerably later than the first differentiation) already partially 

 segmented into rays at br. Outside the region of the cartilaginous 

 plate is seen the fringe with the horny fibres {k.f) ; and dorsalfy and 

 ventrally to the cartilaginous skeleton are seen the already well- 

 differentiated muscles (m). 



The pectoral fin is shown in horizontal section in Plate LVIII. 

 fig. 6, at a somewhat earlier stage than that to which the transverse 

 sections belong. The pectoral girdle (p . g) is cut transversely, and 

 is seen to be perfectly continuous with the basal bar {bp) of the fin. 

 A similar continuity between the basal bar of the pelvic fin and the 

 pelvic girdle is shown in Plate LVII. fig. 2, at a somewhat later stage. 

 The plate continuous with the basal bar of the fin is at first, to a con- 

 siderable extent in the pectoral, and to ?ome extent in the pelvic fin, a 

 continuous lamina, which subseijuently segments into rays. In the 

 parts of the plate which eventually form distinct rays, however, almost 

 from the first the cells are more concentrated than in those parts 

 which will form the tissue between the rays ; and I am not inclined 

 to lay any stress whatever upon the fact of the cartilaginous fin-rays 

 being primitively part of a continuous lamina, but regard it as a 

 secondary phenomenon, dependent on the mode of conversion of em- 

 bryonic mesoblast cells into cartilage. In all cases the separation 



Proc. Zool. Soc— 1881, No. XLIII. 43 



