Watson: The Genus Gyrocotyle. 405 



ovaries of the two examples examined, containing embryos at different 

 stages of development in the uterus, and a curious difference obtains also 

 in the yolk-glands of the two forms. In the one containing highly 

 developed embryos the yolk-glands, like the ovaries, are evidently provid- 

 ing a fresh supply of material in prospect of the next period of repro- 

 ductive activity. Each consists of a mass of cells, the outlines~of~ which 

 are somewhat more clearly marked than in the case of ovaries, with large 

 nuclei evidently undergoing division. The cells are remarkably similar 

 to ova, but the relative size of the yolk masses and their definite super- 

 ficial position renders them distinct from the ovaries. In the case of the 

 form containing the ova passing down into the uterus, the yolk-glands 

 are in a much more advanced stage. Each is filled with a mass composed 

 partly of distinct yellow globular bodies, and partly of nucleated cells. 

 So far as can be seen there are no definite "shell-glands' 7 present; all 

 the other structures connected with the reproductive organs could be 

 distinctly made out by means of sections, and presumably shell-glands 

 would have been able to be recognized if they were present as distinct 

 and separate structures. In plate 13, figure 2, is represented a portion of 

 the first part of the uterus, in which evidently the shells are being 

 formed around the ova. In addition to nucleated cells, the uterus con- 

 tains very numerous little drop-like yellow structures, which resemble 

 exactly those which have been previously described as present in the 

 yolk-glands. It appears as if these, as it were, "ran together," and 

 formed a case enclosing certain of the nucleated cells, some of which are 

 ova, and some probably cells from the yolk-glands which will serve as 

 food for the developing ova. This appears to be the only construction 

 which can be placed upon the appearances." 



Failing to find any shell-glands in G. rugosa, Spencer's 

 hypothesis (though not definitely stated to be such) seems to be 

 that both shell-material and yolk-platelets are formed in the yolk- 

 glands, as indicated in his plate 13. Nothing resembling this 

 type of follicle, or indicating such a function of the vitellaria, 

 has been seen in G. fimbriata. It is hard to believe that Spencer 

 could have observed so delicate and obscure a structure as the 

 network of yolk- ducts in the region of the receptaculum seminis, 

 and overlooked the large and unmistakable* masses of the shell- 

 glands, had they been present in G. rugosa. Lonnberg (1891), 

 who observed the shell-glands in G. urna, suggests that Spencer 

 mistook the central mass of the shell-glands for the central part 

 of the ovary. This suggestion is rendered less plausible by the 

 fact that the main mass of the shell-glands lies, not in the median 

 line, but laterad of the receptaculum seminis, and is simply 

 ma'rked off from the mass of the ovaries. A revision of Spencer 's 

 material seems to be the only way in which the discrepancy in 

 observations can be explained. 



