DEVELOPMENT OF ELASMOBRANCH FISHES. 101 



It seems probable that the central canal of the nervous 

 system arose as an involution from the exterior, and therefore 

 that the epidermis lining it is in reality merely a part of the 

 external epidermis, and as such is naturally separated from the 

 true nervous structures adjacent to it\ 



Leaving the general features of the external skin, I pass 

 to the special organs derived from it during the stage just 

 anterior to K. 



The unpaired Fins. The unpaired fins have grown consider- 

 ably, and the epiblast composing them becomes, like the remainder 

 of the layer, divided into two strata, both however composed of 

 more or less columnar cells. The ventral fin has now become 

 more prominent than the dorsal fin; but the latter extends 

 forward as a fold quite to the anterior part of the body. 



The pah^ed Fins. Along each side of the body there appears 

 during this stage a thickened line of epiblast, which from the 

 first exhibits two special developments : one of these just in front 

 of the anus, and a second and better marked one opposite the 

 front end of the segmental duct. These two special thickenings 

 are the rudiments of the paired fins, which thus arise as special 

 developments of a continuous ridge on each side, precisely like 

 the ridges of epiblast which form the rudiments of the un- 

 paired fins. 



Similar thickenings to those in Elasmobranchs are found at 

 the ends of the limbs in the embryos of both Birds and Mammals, 

 in the form of caps of columnar epiblast ^ 



The ridge, of which the limbs are special developments, is 

 situated on a level slightly ventral to that of the dorsal aorta, 

 and extends from just behind the head to the level of the anus. 

 It is not noticeable in surface views, but appears in sections 

 as a portion of the epiblast w^here the cells are more columnar 

 than elsewhere ; precisely resembling in this respect the for- 

 ward continuation of the dorsal fin. At the present stage the 

 posterior thickenings of this ridge which forms the abdominal 

 fins are so slight as to be barely visible, and their real nature 

 can only be detected by a careful comparison between sections 



^ Vide Self, Development of Spinal Nerves in Elasmobranchs. Phil. Transact. 

 1876. 



3 For Birds, vide Elements of Embryology, Foster and Balfoar, pp. 144—145, 

 and for Mammals, Kolliker Entwicklungsgeschichte, p. 283. 



