STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 821 



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 shewn in Plate 39, 



fig. 55- 



There can be no doubt that this canal is the commencing 

 ovarian sack. 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 39, 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 sack, the outer side of the ovary with the band of 

 thickened germinal epithelium is turned towards the lumen of 

 the sack ; and thus the fact of the ova being formed on the 

 inner wall of the genital sack in the adult is explained, and the 

 comparison which we instituted in our description of the adult 

 between the inner wall of the genital sack 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 sack, 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 im- 

 pression 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 seg- 

 mental 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 



