M. C. Robin on the Sexual Differences of Eels. 391 



The sex being ascertained, the general facts regarding the 

 reproduction of these Apodal fishes follows therefrom ; and 

 these facts do not differ from what they are in nearly all other 

 fishes, the Salmons in particular. Only the propagative 

 migration of the Eels taking place from the fresh waters to 

 the sea, the mode in which oviposition is effected, the fecun- 

 dation and hatching of the ova, are still unknown. The 

 Salmons, behaving in a directly opposite fashion, we have 

 been able, so far as they are concerned, to study and utilize 

 all these physiological peculiarities. 



The same causes have hitherto prevented our seeing the 

 testes of the Eels as they are at their arrival at the state of 

 milt^ and observing their spermatozoids, notwithstanding the 

 abundance of the males (or ^^j^i/jeneaiix). But the period of 

 the descent of the females towards the sea (November) shows 

 that it is in November and December that they ought to be 

 studied. These, however, are the only two months during 

 which I have as yet been unable to observe them. I have 

 ascertained that in October there are as yet no fecundating 

 elements, and that in January there are no longer any. In 

 the Landes and other parts of the south no doubt, the ascent 

 of the young fish taking place as early as the second half of 

 December, instead of in March, as in the Channel, these 

 investigations will have to be made as early as September or 

 October. As to the return of the females from the sea to the 

 fresh waters, this cannot be denied ; in fact I have received 

 from M. Dufourcet some female eels of the variety sardias^ 

 taken in January and February in the Adour at about 

 40 kiloms. from the sea, one half of which had the stomach 

 filled with examples of Eunice scmguinea and Doris, which 

 are exclusively marine invertebrates. 



Except as regards the minute structural determination and 

 the truly testicular nature of the organ homologous witli the 

 ovaries, the preceding anatomical data are not new. The 

 want of this determination and of the observation of the sper- 

 matozoids is probably what has led to their not having hitherto 

 been taken into consideration as they deserve to be. 



Duvernoy (Cuvier, Anatomic Comparce, ed. 2, 1846, 

 tome viii. p. 117) describes the ruffle-like type of the testis of 

 the Lampreys and Eels, with the free margin festooned in 

 lobules, shorter to the right than to the left, like the ovaries, 

 &c. He adds : — " At the breeding-season we perceive in it 

 an innumerable quantity of granulations or small spermatic 

 capsules, the rounded form of which has often led to their 

 being confounded with the ovules, at least in the Eels, in 

 which, in reality, these capsules are of nearly the same size as 



29* 



