ANNUAL OVARIAN CYCLE 



Between the "zona pellucida," or "zona radiata," if the latter is present, 

 and the yolk region is an area of yolk-free cytoplasm (See Fig. 4.) This is 

 seen in oocytes of all periods of development. Within this, in those stages 

 where yolk is present, lies first a narrow region of very fine yolk, and then 

 a dense layer of cytoplasm filled with large yolk spherules; the region im- 

 mediately surrounding the nucleus carries less yolk. The comparative density 

 of these regions varies during development. The cytoplasmic portion is ap- 

 proximately two-thirds the diameter of the entire oocyte. This proportion is 

 less, of course, in the smaller oocytes where yolk is either lacking entirely or 

 present in small quantities. (See Figs. 2-4.) 



The nuclear membrane of the oocytes is seen distinctly in all series 

 studied. The shape of the nucleus is in general oval, although quite irregular 

 in the larger oocytes. Darkly staining nucleoli are present in varying numbers 

 in the nucleus of the smallest oocytes found in ovaries of mature specimens 

 of Acris and persist throughout the growth period. (See Figs. 4 and 6.) The 

 origin of these nucleoli has not been adequately explained by those who 

 have described them for other forms. Pratt and Long (1917), studying the 

 egg of the white rat, Mus norvcgicus albinus, thought these "chromatin 

 nucleoli," as they have been designated in some instances, to be merely closely 

 compacted clumps of chromatin which have migrated back along the linin 

 threads to one or more common points. An adequate determination of their 

 origin and function in these oocytes necessitates further work. Their staining 

 capacity is the same as that of the chromosomes, however. The nucleoli vary 

 in size, and, as noted earlier, change their position relative to the chromo- 

 somes as growth proceeds from month to month. 



The behavior of the chromosomes during the period of growth, as sug- 

 gested above, is of little interest. The chromosomes stain very lightly and are 

 inconspicuously grouped in the center of the nucleus, apparently in more or 

 less reticular form, but in some instances they have a feathery appearance. 



Monthly Changes in the Ovaries 

 Ovaries of frogs collected and fixed during September are of such char- 

 acter as definitely to mark the end of the old cycle and the beginning of the 

 new. (See Fig. 2.) Few evacuated or degenerating follicles are present. The 

 cytoplasm of the small oocytes is undifferentiated, lacking in yolk and "yolk 

 nuclei." In the average oocyte of frogs of September, the cytoplasm occupies 

 no more than half the diameter of the entire oocyte, while in the months just 

 passed the proportion of cytoplasm is markedly greater. The follicular mem- 

 brane is present, of course, but the "zona pellucida" is seen around only the 

 very largest of the oocytes. A pigmented membrane bearing what seems to 

 be particles of fat often lies beside the ovary. This is doubtless a portion of 

 the mesovarium. The position of the chromatin nucleoli is distinctly different 

 from that found in oocytes of frogs collected during the active breeding 

 months. The nucleoli here are sparse and lie close to the nuclear membrane; 

 none are found in the central region of the nucleus, as in the ovaries of frogs 

 during the summer. 



