102 THE NATURE OF THE PAIRED FINS. 



of this and the succeeclmg stages. The rudiments of the anterior 

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

 though the passage between them and the remainder of the 

 ridofes is most grradual. 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 develope into an elong- 

 ated 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. xi. 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, develope 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 develope 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. 



