220 METAMORPHOSIS AND 



tubercles of the Flounder, and the scattered thorn- 

 like tubercles of the Turbot, develop directly, not by 

 the continuous modification of imbricated scales. 

 There is, however, one scale-character among the 

 Pleuronectidae which appears to stand in direct con- 

 tradiction to the conclusions drawn by me concerning 

 scales in general. It not only develops by a gradual 

 change, but it is a secondary sexual character de- 

 veloping in the males only at maturity. The char- 

 acter was described by E. W. L. Holt in specimens 

 of the Baltic variety of the Plaice, Pleuronectes 

 platessa,^ and consists in the spinulation of the 

 posterior edges of the scales, especially on the upper 

 side, in mature males. The same condition, but to a 

 much slighter degree, was afterwards shown by my- 

 self to occur constantly in Plaice from the EngUsh 

 Channel and North Sea.^ It occurs also in P. 

 glacialis, the representative of the Plaice in more 

 northern seas. I have shown that the spinules 

 develop in the mature males not as a modification 

 of the scale, but as separate calcareous deposits the 

 bases of which afterwards become united to the 

 scale. It would seem that the development of this 

 character is dependent on the hormone from the 

 mature testis, and in order to conform with the 

 arguments used by me in other cases, the spinulation 

 should have some definite function in relation to the 

 habits of the sexes, and this function should involve 

 some kind of external stimulation restricted to the 

 mature male. So far, however, no evidence what- 

 ever of such function or such stimulation has been 

 discovered. It is possible that the case differs from 



^ Journ, Mar. Biol Assn., vol. iii. (Plymouth, 1893-95). 

 2 Ihid., vol. iv. p. 323, 



