136 PROF. HUXLEY ON THE OVIDUCTS OF OSMERUS. [Mar. 20, 



middle, while below it is bounded by a smooth thin-edged band 

 invested by peritoneum (fig. 1, B). A transverse section shows that 

 the ovigerous lamellae pass under this reflected band (to which their 

 outer edges are attached) to the ventral wall of the ovary. The 

 groove inclosed by the reflected band is thus divided into a series of 

 interlamellar loculi (fig. 1, C). 



Thus far the ovary agrees in all the essentials of its structure with 

 that of the other Salmonidge, and with that of all adult Ganoids 

 except Lepidosteus. Even in the latter, Balfour has shown that the 

 ovary passes through a similar condition in the embryonic state. 

 The mesoarium, however, does not stop at the posterior end of the 

 ovary, but, as Rathke points out, the fold of peritoneum which 

 constitutes it is continued backwards to the oviducal aperture ; while 

 laterally it passes into the peritoneal lining of the lateral walls of 

 the abdomen, ending in a free concave edge immediately behind and 

 on the outer side of the posterior extremity of the ovary. It thus 

 forms the ventral boundary of a passage which opens in front by a 

 wide ostium into the abdominal cavity (fig. 1, od. I, od.r). As the 

 posterior end of the right ovary lies very far behind the posterior 

 end of the left ovary, it follows that the right ostium is equally far 

 behind the left, and that the right passage is by so much shorter 

 than the left. The mesentery terminates by a free posteriorly con- 

 cave edge (which contains the rectal artery) just opposite the level 

 of the posterior end of the right ovary ; and, behind this free concave 

 edge of the mesentery, the right and left passages unite in a short but 

 wide common chamber, which opens externally in the middle line, 

 behind the anus and in front of the urinary outlet (fig. 1,$'). 



In a Smelt ready to spawn, these passages, as well as the common 

 chamber, are crammed full of ova ; and it is obvious that, whatever 

 their morphological nature, they are, in a physiological sense, ovi- 

 ducts, comparable to Fallopian tubes. 



But every one who is familiar with the anatomy of the female 

 reproductive organs of the Ganoids, will at once perceive that these 

 passages are the homologues of the oviducts of Acipenser, Polyodon, 

 Polypterus, and Amia (fig. 2, p. 137). Neither in structure, nor in 

 their essential anatomical relations, is there any difference between 

 them. It is true that, in the Ganoids iti question, the oviducts com- 

 municate with the renal ducts, and that the excretory aperture is com- 

 mon to the urinary and the genital apparatus, while in the Smelt 

 there is no such communication and the oviducal and renal apertures 

 are separate. But, among the Sturiones and in Lepidosteus, the 

 renal are much wider than the genital ducts, and the communication 

 between the two is effected far in front of the external aperture, 

 while in Polypterus and Amia the oviducts are much wider than 

 the ureters and the communication takes place near the external 

 aperture. Thus the arrangement in Osmerus represents simply the 

 third term of a series of modifications, tending towards the separation 

 of the ureteric from the oviducal ducts, two terms of which are pre- 

 sented by the Ganoids. And it follows that the arrangement of the 

 parts which obtains in the ordinary Salmonidse is a fourth term in 



