STRUCTURE AND DEVELOPMENT OP LEPIDOSTEUS. 419 
it was attached on the level of the outer side of the segmental duct. A somewhat 
triangular channel was thus constituted, the inner wall of which was formed by the 
ovary, the outer by the lamina just spoken of, and the roof by the strip of the peri¬ 
toneum of the abdominal wall covering that part of the ventral surface of the kidney 
in which the openings of the peritoneal funnels of the excretory tubes are placed. 
The structure of this canal will be at once understood by the section of it shown in 
Plate 26, fig. 55. 
f There can be no doubt that this canal is the commencing ovarian sac. On tracing 
it backwards we found that the lamina forming its outer wall arises as a fold growing 
upwards from the free edge of the genital ridge meeting a downward growth of the 
peritoneal membrane from the dorsal wall of the abdomen; and in Plate 26, fig. 56, 
these two laminae may be seen before they have met. Anteriorly the canal becomes 
gradually smaller and smaller in correlation with the reduced size of the ovarian ridge, 
and ends blindly nearly on a level with the front end of the excretory organs. 
It should be noted that, owing to the mode of formation of the ovarian sac, the 
outer side of the ovary with the band of thickened germinal epithelium is turned 
towards the lumen of the sac ; and thus the fact of the ova being formed on the 
inner wall of the genital sac in the adult is explained, and the comparison which we 
instituted in our description of the adult between the inner wall of the genital sac 
and the free genital ridge of Elasmobranchs receives its justification. 
It is further to be noticed that, from the mode of formation of the ovarian sac, the 
openings of the peritoneal funnels of the excretory organs ought to open into its 
lumen ; and if these openings persist in the. adult, they will no doubt be found in 
this situation. 
Before entering on further theoretical considerations with reference to the oviduct, it 
will be convenient to complete our description of the excretory organs at this stage. 
When we dissected the excretory organs out, and removed them from the body of 
the young Fish, we were under the impression that they extended for the whole length 
of the body-cavity. Great was our astonishment to find that slightly in front of the 
end of the ovary both excretory organs and segmental ducts grew rapidly smaller and 
finally vanished, and that what we had taken to be the front part of the kidney was 
nothing else but a linear streak of tissue formed of cells with peculiar granular con¬ 
tents supported in a trabecular work (Plate 26, fig. 54). This discovery first led us to 
investigate histologically what we, in common with previous observers, had supposed 
to be the anterior end of the kidneys in the adult, and to show that they were nothing 
else but trabecular tissue with cells like that of lymphatic glands. The interruption 
of the segmental duct at the commencement of this tissue demonstrates that if any 
rudiment of the pronephros still persists, it is quite functionless,, in that it is not 
provided with a duct. 
