STAGES G TO K. THE PAIRED FINS. 321 



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 fins. If it be taken into consideration that the 

 continuous lateral fin, of which the rudiment appears in Elasmo- 

 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 fins 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 hypothesis here adopted 1 . 



My researches throw no light on the nature of the skeletal 

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

 Giinther 2 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. 



Dr Dohrn 8 in speaking of the limbs, points out the difficulties 



1 For the nervous supply in fishes, vide Stannius, Peripher. Nerv. 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 Acipenser there are 

 branches from the first six nerves. In Spinax the limb is supplied by the rami 

 anteriores of the fourth and succeeding ten spinal nerves. In the Rays not only do 

 the sixteen anterior spinal nerves unite to supply the fin, but in all there are rami 

 anteriores from thirty spinal nerves which pass to the thoracic limb. 



2 Philosophical Transactions, 1871. 



3 Ursprung d. Wirbelthiere nnd Fnnctivnsivechsels. 



