482 EDWIN S. GOODHIOH. 



jointed region is formed cliiefly of the harder outer layer ; 

 the distal region, on the contrary, cliietiy of the softer inner 

 layer. 



Protopterus is provided with dermal rays of essentially 

 the same structure. They differ, however, in being smaller, 

 softer, less calcified, less frequently jointed and branched, 

 and in having fewer cells included in their substance. 



In Lepidosiren, again, the dermal rays, although of the 

 same nature, are still further reduced. They have dis- 

 appeared altogether from the paired fins, and in the unpaired 

 fins are shorter, very soft, scarcely jointed, and uubranched. 



Protopterus is the only Dipnoan in which I have been able 

 to study the development of the dermal fin-rays.^ Sections 

 through the median fins show the usual basement membrane 

 separating the epidermis from the underlying mesoblastic 

 tissues. Towards the growing edge of the fin, immediately 

 below the basement membrane, can be seen the early stages 

 in the development of the ray, in the form of a delicate 

 thread of cuticular substance secreted by rows of scleroblasts 

 (figs. 13 and 14). The distal extremity of the thread does 

 not appear to originate distinctly as a mere thickening of the 

 membrane, but becomes smaller and smaller on passing out- 

 wards, and finally becomes indistinguishable from a network 

 of connective-tissue fibres underlying the basement mem- 

 brane. As it grows, the ray sinks away from the membrane 

 aud becomes surrounded by the secreting scleroblasts. Very 

 soon the distinction between the outer and inner regions can 

 be made out (figs. 15 and 16). A fairly definite sheet of 

 connective tissue joins the dermal rays together, some dis- 

 tance below the epidermis (fig. 16). Meantime the scales 

 develop on the body and spread over the proximal region of 

 the fin, on the outer surface of the rays. 



Neither in adult Dipnoi, nor in the larva of Protopterus, 

 have I fouud any trace of actiuotrichia. It is ])ossible that 



' Mr. J. S. Budf^ett very kindly presented me with two iarvec of Proto- 

 pterus, on wliicli these observations iiave been made. 



