DEVELOPMENT OF ELASMOBRANCH FISHES. 1(3 



The development of the limbs is almost identically similar 

 to that of the dorsal fins. There appears a lateral linear thick- 

 ening of epiblast, which however does not, like the similar 

 thickening of the fins, grow into a distinct fold. Its develop- 

 ment becomes confined to two special points, at each of which 

 is formed a continuous elongated fold of columnar cells precisely 

 like the fold of skin forming the dorsal fins. These two folds 

 form the paired pins. If it be taken into consideration that the 

 continuous lateral fin, of which the rudiment appears inElasmo- 

 branchs, does not exist in any adult Vertebrate, and also that a 

 continuous dorsal fin exists in many Fishes, the small differences 

 in development between the paired fin and the dorsal fins will 

 be seen to be exactly those which might have been anticipated 

 beforehand. Whereas the continuous dorsal fin, which often 

 persists in adult fishes, attains a considerable development before 

 vanishing, the originally continuous lateral one has only a 

 very ephemeral existence. 



While the facts of development strongly favour a view which 

 would regard the limbs as remnants of a primitively continuous 

 lateral fin, there is nothing in the structure of the limbs of 

 adult Fishes which is opposed to this view. Externally they 

 closely resemble the unpaired fins, and both their position and 

 nervous supply appear clearly to indicate that they do not 

 belong to one special segment of the body. They appear 

 rather to be connected with a varying number of segments ; a 

 fact which would receive a simple explanation on the hypo- 

 thesis here adopted \ 



My researches throw no light on the nature of the skeletal 

 parts of the limb, but the suggestion which has been made by 

 Giinther^ with reference to the limb of Ceratodus (the most 

 primitive known), that it is a modification of a series of parallel 

 rays, would very well suit the view here proposed. 



1 For the nervous supply in fishes, vide Stannius Periplier. Kerv. System d. 

 Fische. In Osseous Fishes he states that the thoracic fin is supplied by branches 

 from the first three though sometimes from the first four spinal nerves. In 

 Accipenser there are branches from the first six nerves. In Spinax the limb is 

 supplied by the rami anteriores of the fomih and succeeding ten spinal nerves. 

 In the Ptays not only do the sixteen anterior spinal nerves unite to supply the 

 fin, but in all there are rami anteriores from thkty spiual nerves which pass to 

 the thoracic limb. 



2 Philosophical Trajisactions, 1871. 



