OF THE PAIRED FINS OF ELASMOBRANCHS. 725 



Development of the paired Fins. The first rudiments of the 

 limbs appear in Scy Ilium, 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 in 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 con- 

 nected ; but in some Elasmobranch embryos, more especially in 

 that of Torpedo, they are connected together at their first develop- 

 ment 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 

 edge of which is a fold of epiblast only, which soon reaches con- 

 siderable 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, 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 shewn, 

 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 shewn 

 in section in Plate 33, fig. I m, and in Plate 33, fig. 7 m. 



The cartilaginous skeleton of the limbs is developed in the 

 indifferent mesoblast cells 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 



