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



Murcence * and of various otlier fishes ; they may even be 

 compared with those which exist between the testes and the 

 ovaries in the Batrachia and Birds. 



But these external sexual differences are only sufficient 

 because they correspond to structural dissimilarities existing 

 between the constituent elements of the internal organs, ful- 

 filling the physiological function of male on the one hand, 

 of female on the other, and this in spite of morphological, 

 embryogenic homologies and of connexions which approxi- 

 mate the ovary to the testis. This is what histology shows 

 incontestably. 



The want of this determination of the minute structure of 

 these organs has caused people not to give to the external 

 characters (those of the pimpeneaUy for example) the impor- 

 tance they possess as belonging here to the male, elsewhere 

 to the female, with the body more swelled and not so black, 

 the head more produced, the eye smaller, &c. This gap has 

 even prevented some anatomists from taking count of the 

 external differences which, at all seasons, exist between the 

 ovary and the testis, the differences of structure of which are 

 also always perceptible under the microscope. 



On the one hand, at any period of the year, the ovary 

 shows its ovules, more or less developed, but like those of all 

 other osseous fishes, and its loose cellular tissue, which maj be 

 reduced to a minimum towards the period of oviposition, or, 

 on the contrary, become in part cellulo-adipose subsequently ; 

 the ovary also always shows the narrow projections or thick- 

 enings of the surfaces of its lobes, parallel to each other, re- 

 sembling folds running from the adherent to the free margin, 

 and passing the latter in the form of small blunt denticu- 

 lations. 



On the other hand, the lobulate testis, of firmer consistence, 

 with a close cellular texture, without adipose cells, traversed 

 throughout its whole extent by seminiferous or testicular tubes^ 

 or cylinders^ which are flexuous, twisted, terminated ca3cally 

 at both ends, at least out of the breeding-season — that is to say, 

 falling into the type of canaliculate testes, such as that of the 

 carps. 



The contents of these tubes, which answer to what are called 

 spermatic capsules in the case of other fishes, render the testis 

 opalescent whitish grey, instead of the reddish-grey tint which 

 is usual when its vessels are congested, a fact connected with 

 the absence of ovules more or less rich in yellowish oily drops. 

 These contents render the male organ more or less white, and 



* See C. Robiri " Sur le coeur caudal des Anguilles/' Journ. Anat. 

 Physiol. 1880, p. 597. 



