or THE PAIRED FINS OF ELASMOBRANCHS. 61 



being single as is the case with the remaining rays. It is 

 probably equivalent to two of the posterior rays. 



Development of the paired Fins. — The first rudinoents of 

 the linabs appear in Scyllium, as in other fishes, as slight 

 longitudinal ridge-like thickenings of the epiblast, which 

 closely resemble the first rudiments of the unpaired fins. 



These ridges are two iu number on each side — an anterior 

 immediately behind the last visceral fold, and a posterior on 

 the level of the cloaca. In most Fishes they are in no way 

 connected ; but in some Elasmobranch embryos, more especi- 

 ally in that of Torpedo, they are connected together at their 

 first development by a line of columnar-epiblast cells. This 

 connecting line of columnar epiblast, however, is a very 

 transitory structure. The rudimentary fins soon become 

 more prominent, consisting of a projecting ridge both of 

 epiblast and mesoblast, at the outer ecige of which is a fold 

 of epiblast only, which soon reaches considerable dimensions. 

 .At a later stage the mesoblast penetrates into this fold, and 

 the fin becomes a simple ridge of mesoblast covered by 

 epiblast. The pectoral fins are at first considerably ahead of 

 the pelvic fins in development. 



The direction of the original epithelial line which connected 

 the two fins of each side is nearly, though not quite, longi- 

 tudinal, 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 

 iinib, which are shown in section in Plate IV, fig. 1 m and in 

 Plate V, fig. 7 m. 



The cartilaginous skeleton of the limbs is developed in the 

 indifferent 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 histolo- 

 gically 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 pec- 



