420 



PEOF. B. T. LOWNE OlST THE STRUCTTJRE AND 



divergeDce of opiDion ; the identification of its nucleus with the 

 germinal vesicle has been regarded as of the highest importance, 

 and numerous attempts have been made to show that the nucleus 

 of the egg-cell differs from those of the nutrient cells, from the 

 very earliest stages of the egg-formation. Thus Dr. Claus (8) 

 (p. 44) writes : — " The questions, tlie answers to which are of the 

 highest importance, are : from whence is the germinal vesicle 

 derived ? and what is its relation to the great yelk-cells ? Tiie 

 identification of the germinal vesicle appears to be difficult, and 

 the earlier observers came to no certain result." After quoting 

 from Sir John Lubbock's paper (19) he continues, " 1 believe my 

 o^vn observations enable me to prove that the epithelial cells, the 

 yelk-forming cells, and the egg-cell are modifications of originally 

 identical elements." Yet Claus thinks he distinguishes the 

 germinal vesicle in very immature ova by " its smaller size and 

 clearer contents " from the nuclei of the adjacent yelk-cells. 

 Meyer's (20) statements are in direct opposition to this ; according 

 to these there are several germinal vesicles, each nucleus becomes 

 invested by a membrane, each making, as it were, an attempt at 

 forming an egg, the lowest nucleus persisting and the others 

 disappearing. 



In my opinion by far the best and most accurate description of 

 the development of the ovarian eggs in the Ply is due to Weis- 

 mann (26) ; it is as follows : — " The ovaries are developed very 

 slowly in the Muscidae ; when the fly emerges from the pupa 

 none of the eggs are formed, although the ovary may be recog- 

 nized in the embryo" (Z. c. p. 206). Weismann then refers to 

 a description of the ovary in the adult larva of the closely-allied 

 SarcopJiaga carnaria (at page 134), " Concerning the female sexual 

 organs in the larva of Musca vomitoria I have no observations ; 

 I must therefore fall back on those on the closely-allied >S'. car- 

 naria. In a larva one centimetre long, the ovary has a diameter of 

 0*29°', is flask-shaped, and differs in its histological structure from 

 the male sexual gland ; here w^e find no mother-cells, the struc- 

 tureless sheath encloses only small round cells 'OlS"" in diameter 

 wilh nuclei 'Ol"", exceedingly clear and exhibiting nucleoli. These 

 cells are isolated with difficulty, as they adhere closely to each 

 other. If we tear the ovarium of an adult larva no further 

 structure is visible; but if a gentle pressure be applied to the 

 uninjured ovary one may distinguish the first rudiments of the 

 ovarian tubes. In the upper half of the ovary they appear as 



