820 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



ducts coalesce to form the urinary bladder, the genital ridge 

 comes into view. For its whole extent, except near its anterior 

 part (of which more hereafter) this ridge projects freely into the 

 body-cavity, and in this respect the young Fish differs entirely 

 from the adult. As shewn in Plate 39, figs. 56 and 57 (g.r.), it 

 is attached to the abdominal wall on the ventral side of, and near 

 the inner border of each kidney. The genital ridge itself has a 

 structure very similar to that which is characteristic of young 

 Elasmobranchii, and it may be presumed of young Fishes 

 generally. The free edge of the ridge is swollen, and this part 

 constitutes the true generative region of the ridge, while its dorsal 

 portion forms the supporting mesentery. The ridge itself is 

 formed of a central stroma and a germinal epithelium covering 

 it. The epithelium is thin on the whole of the inner aspect of 

 the ridge, but, just as in Elasmobranchii, it becomes greatly 

 thickened for a band-like strip on the outer aspect. Here, the 

 epithelium is several layers deep, and contains numerous primitive 

 germinal cells (p.o.}. 



Though the generative organs were not sufficiently advanced 

 for us to decide the point with certainty, the structure of the 

 organ is in favour of the view that this specimen was a female, 

 and, as will be shewn directly, there can on other grounds be no 

 doubt that this is so. The large size of the primitive germinal 

 cells (primitive ova) reminded us of these bodies in Elasmo- 

 branchii. 



In the region between the insertion of the genital ridge (or 

 ovary, as we may more conveniently call it) and the segmental 

 duct we detected the openings of a series of peritoneal funnels of 

 the excretory tubes (Plate 39 , fig. 57, /./!), which clearly there- 

 fore persist till the young Fish has reached a very considerable 

 size. 



As we have already said, the ovary projects freely into the 

 body-cavity for the greater part of its length. Anteriorly, how- 

 ever, we found that a lamina extended from the free ventral 

 edge of the ovary to the dorsal wall of the body-cavity, to which 

 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 peritoneum 



