298 MR. E. W. L. HOLT ON TUB L^W' ^^» 



of " guauinkalk," ' and the coloration elements of the skin have 

 been investigated, in each case through a series of species of 

 Teleosteans, by Ewald and Krukenberg ", and by Cunningham and 

 MacMuun \ 



So far as I am awai-e, the histological and physiological changes 

 involved in the sexual colour-differentiation of fisbes have received 

 but little attention. In fact, I believe that Heincke \ in his 

 observations on Gohius ruthensparri, is the only contributor to 

 this subject. In the goby some of the distinctions which Heincke 

 supposed to be sexual have been shown by Guitel ' to be in some 

 degree common to both male and female, and, although the male 

 is certainly the more brilliant and especially at the breeding- 

 season, the differences of coloration, except such as affect some of 

 the fins, are only those of degree. The sexual colour-differentia- 

 tion of Callionynms is infinitely more striking, and since my 

 observations bring out some points not touched upon by Heincke, 

 it appears worth while to put them forward even iu their present 

 imperfect condition. I hope to find time to complete them, and 

 to include in my inquiry such other sexually dimorphic forms as 

 may be procurable. 



Pouchet's term " iridocyte," applied to plate-like aggregations 

 of the reflecting substance which show some traces of a cellular 

 nature or origin, has been retained by Cunningham and MacMunu, 

 who apply a new term, " argenteum,'' to a laj'er of particles of 

 similar substance which usually constitutes the most deeply-seated 

 element of the colour-mechanism. 



The authors show that the reflecting substance of the outer 

 layer is not always found in the form of definite plate-like bodies, 

 but may be present in minute particles of variable shape, which 

 apparently do not always differ from the particles composing the 

 argenteum except in their topographical relations. It was found 

 that the reflecting substance usually consisted of guanin, though 

 calcium phosphate was present in some species. 



1 cannot at present deal with the chemical nature of the colour- 

 elements in the Dragonet, and must therefore confine myself to a 

 preliminary discussion of their disposition and sexual and develop- 

 mental differentiation. 



Since the young male and the female are identical in coloration, 

 it is only necessary to compare the several colour-phases of the 

 former sex. The Di'agouet is said to have no scales, and a minute 

 histological examination of the skin sho\\s no obvious trace of 

 such structures. It is most abundantly supplied with mucus, 

 which it throws off in slimy clouds when irritated. The secretion 



^ Kiihne u. Sftwall, Uutersucb. Plijs. Inst. Heidelb. iii. p. 221. 



2 Zeitschv. f. Biol. xix. 1883. 



^ Phil. Traus. R. S. clxxxiy. 1894-, p. 765. I am indebted to these authors 

 for most of tbe above and for other references. 



* Schr. Naturw. Ver. Schlesw. Hoist. 1 875, i. p. 290. A full translation is 

 given by Smitt, Hist. Scand. Fish. ed. 2, i. p. 242. 



« Arch. Zool. Exper. ser. 3, iii. 1898, p. 264. 



