ON THE ANATOMY OV HISTRIOBDELLA HOMAR[. 335 



ampulla to the exterior. When the ovary is full of ova it is 

 sometimes difficult to see the funnel and oviduct, as its 

 lumen is obliterated by compression against the body-wall. 



The inner surface of the ovary is closely invested with a' 

 thin layer of nucleated cells — the true coelomic epitlielium. 

 It is from this in the anterior region that the primitive ova 

 arise. This takes place close to where the sac abuts against 

 the end of the third segment. Here certain of the nuclei are 

 much larger than the rest. They are the oogonial cells. 

 They have relatively little cytoplasm and. large, transparent 

 nuclei. A considerable number of them are seen at this 

 point in different stages of development. The fact that the 

 oogonia arise from a small, circumscribed portion of the 

 anterior end of the ovary, and not from its epithelial surface 

 in general, recalls the condition described by Nelson (25) in 

 Dinophilus conklini, which differs from the other species 

 of this group, D. vorticoides, D. taeniatus, andD. gigas, 

 in that only a small portion of the ovary likewise gives rise 

 to the oogonial cells. It is evident that the epithelium of the 

 middle and posterior portions of the ovarian cavity play no 

 part in their formation. As they pass backwards and become 

 the primary oocytes, the epithelium of this part of the cavity 

 throws out processes that attach themselves to the growing 

 oocytes, folding up round them and forming a supporting- 

 matrix crowded with small nuclei. They furnish them with 

 the material for their growth, but beyond this take no part in 

 their formation. As the oocyte grows these follicle cells 

 diminish rapidly in size, and their nuclei undergo degenera- 

 tion, becoming long and granular. They appear to have 

 something to do with the formation of the yolk-granules, but 

 how this is accomplished is not plain. These arise in situ, 

 as nothing similar to them can be distinguished in the follicle 

 cells, which are always clear and transparent. At the time 

 of their formation the granules are also clear and. transparent, 

 and only acquire their dark appearance after they have been 

 formed some time. For this reason the [small oocytes, 

 although highly granular, are almost as transparent as the 



