ANNUAL OVARIAN CYCLE 



preceding. The size range during the month is approximately the same as 

 during March, although there seem to be fewer very small oocytes. Little 

 change is seen in the appearance of the cytoplasm; it stains darkly with the 

 exception of the region immediately surrounding the nucleus. 



The average size of the oocytes of frogs collected in May increased defi- 

 nitely, for very small ones are fewer in number, and the larger ones have 

 accumulated yolk near their periphery. Here, as in other months, the larger 

 oocytes are found in the center of the ovaries. The chromatin nucleoli at this 

 period lie very close to the chromosomes and vary considerably in size. 



The average size of the oocytes is increased still more in ovaries of frogs 

 fixed in June. The majority are yolk-laden and in an advanced state of 

 growth. Very few small oocytes are present. No change was noted in the ap- 

 pearance of the chromosomes from those of the preceding months. 



It is necessary in describing ovaries of July frogs to consider two stages 

 of development, that of frogs collected early in July (see Fig. 7) and that of 

 frogs collected late in the month. The former resembles very closely that of 

 specimens fixed in the preceding month, the only difference being a slight 

 increase in size of the oocytes at the later date. Few small oocytes are present. 

 Ovaries of frogs collected late in July, however, contain comparatively few 

 large oocytes, many young ones, a considerable number of follicles from 

 which the oocytes have escaped, and others in which a process of degenera- 

 tion is taking place. Degeneration of the follicular membrane begins shortly 

 after the escape of the oocyte. The cells forming the membrane which sur- 

 rounds the mature oocyte before its escape move into the evacuated follicle 

 in an irregular manner. A similar condition was found in oocytes which 

 were undergoing degeneration, as described below. The collection and study 

 of ovaries of frogs fixed at more frequent intervals during this period should 

 prove to be of much interest in respect to the specific manner in which de- 

 generation of the follicular sac in this form occurs. King (1902) reported 

 the manner of degeneration of the follicular sac in specimens of Bufo. This 

 degeneration must occur, of course, to make room for the new generation 

 of oocytes. Left in the follicle after an oocyte escapes is a reticular framework 

 and in some instances the follicle cells which are moving in from the broken 

 follicular membrane. The "zona pellucida" is not seen, of course, since it be- 

 comes the "vitelline membrane" of the egg in later development and remains 

 with the oocyte as it leaves the ovary. 



In those follicles in which the oocyte is degenerating (see Fig. 8) the 

 reticular framework is present, supporting the remaining yolk which varies 

 in amount with the stage of degeneration. Degeneration seems to occur first 

 in the outer regions of the oocyte, and the last remaining portion of the 

 oocyte is found in the very center of the follicle. The presence of follicles 

 undergoing degeneration substantiates the theory of Hargitt (1930) drawn 

 from his study of the sex glands and germ cells of the female of the albino 

 rat, that many ova degenerate and never come to ovulation. 



The ovaries of frogs collected during August (see Fig. 8) resemble in 

 all general respects those just described for frogs of late July. The number 



