320 DEVELOPMENT OF ELASMOBRANCH FISHES. 



limbs are more visible than those of the posterior, though the 

 passage between them and the remainder of the ridges is most 

 gradual. Thus at first the rudiments of both the limbs are 

 nothing more than slight thickenings of the epiblast, where its 

 cells are more columnar than elsewhere. During stage K the 

 rudiments of both pairs of limbs, but especially of the anterior 

 pair, grow considerably, while at the same time the thickened 

 ridge of epiblast which connects them together rapidly disappears. 

 The thoracic limbs develop into an elongated projecting fold of 

 epiblast, in every way like the folds forming the unpaired fins ; 

 while at the same time the cells of the subjacent mesoblast 

 become closely packed, and form a slight projection, at the 

 summit of which the fold of the epiblast is situated (PI. II, fig. 

 9). The maximum projection of the thoracic fin is slightly 

 in advance of the front end of the segmental duct. The 

 abdominal fins do not, during stage K, develop quite so fast 

 as the thoracic, and at its close are merely elongated areas 

 where the epiblast is much thickened, and below which the 

 mesoblast is slightly condensed. In the succeeding stages 

 they develop into projecting folds of skin, precisely as do the 

 thoracic fins. 



The features of the development of the limbs just described, 

 are especially well shewn in Torpedo ; in the embryos of which 

 the passage from the general linear thickening of epiblast into 

 the but slightly better marked thickening of the thoracic fin 

 is very gradual, and the fact of the limb being nothing else than 

 a special development of the linear lateral thickening is proved 

 in a most conclusive manner. 



If the account just given of the development of the limbs is 

 an accurate record of what really takes place, it is not possible to 

 deny that some light is thrown by it upon the first origin of the 

 vertebrate limbs. The facts can only bear one interpretation, 

 viz. : that the limbs are the remnants of continuous lateral fins. 



The unpaired dorsal fin develops as a continuous thickening, 

 which then grows up into a projecting fold of columnar cells. 

 The greater part of this eventually atrophies, but three separate 

 lobes are left which form the two dorsal fins and the upper lobe 

 of the caudal fin. 



The development of the limbs is almost identically similar 



