THE DKRMAL FIN-RAYS OF FISHES. 



485 



and are often converted into spines by the suppression of the 

 joints. 



The development of the lepidotrichia is somewhat different 

 from that of the ceratotrichia. They appear as thickenings of 

 the basement membrane of considerable width, secreted by 



efr 



Fig. 2. — Diagrammatic figure tiiroiigii the dorsal fin of Lepidosteus. 

 For the lettering see p. 517. 



undei'lying- scleroblasts. Later they become separated off 

 from the membrane as thin plates of bony substance, which 

 sink inwards and become surrounded by the mesoblastic 

 tissues. In forms like Polypterus and Lepidosteus, with 

 ganoid scales, the segments of the dermal rays acquire the 

 same histological structui-e as the body-scales; they have the 

 same covering of ganoine and the same minute denticles on 



