ANNUAL OVARIAN CYCLE 



Specimens of Acris fixed during July and August have ovaries which 

 vary little in external appearance from those of June. Ovulation occurs dur- 

 ing late July, August, and September, and it may be of interest to note the 

 general features of an oocyte at the period just preceding ovulation. When 

 removed from the ovary and observed beneath the dissecting microscope, 

 the typical oocyte is tountl to be pigmented on approximately two-thirds of 

 its surface; the remainder is much lighter in color, a manner of pigmenta- 

 tion which gives the ovaries a speckled appearance. When dissected, the 

 oocyte is found to be covered by a thick membrane, the egg membrane or the 

 "zona pellucida." The yolk layer is approximately one-third the diameter of 

 the ovum in thickness. Due to the great abundance of yolk within this por- 

 tion of the egg, it is thick and opaque. The nucleus, located in approximately 

 the center of the oocyte, has a white, gelatinous appearance. 



Microscopical Structure of the Ovaries 



When ovaries are studied microscopically and in section, both shape and 

 size are found to vary markedly at different seasons of the year. At no time 

 does an ovary seem to have a definite form, but it is made up of a varying 

 number of irregularly shaped lobes. The size of the ovary is determined, of 

 course, by the degree of development of the oocytes contained therein. 



The entire ovary is covered by the mesovarium, which attaches it to the 

 body wall. This membrane is thin, transparent, and dotted by numerous 

 deeply staining spots of pigment. The mesovarium is seldom seen in sections, 

 and is apparently destroyed during the preparation of the ovary for staining. 



The wall of the ovary is double, the two layers being made up of small 

 cells whose boundaries are not visible, but whose nuclei are deeply stained 

 by hematoxylin. Between these layers lie scattered blood vessels which supply 

 the ovary with blood. 



The developing oocytes, with their membranes, fill practically the entire 

 volume of the ovary. A few groups of cells, similar in all respects to those 

 composing the outer ovarian wall, are found between the follicles. Due to 

 their crowded condition, the oocytes are irregularly polygonal in shape, with 

 the exception of those in the outer portion of the ovary, which are rounded 

 on the outer side. 



In mature females of Acris the ovaries contain only germ cells in the 

 period of growth preceding the first maturation division. No oocytes with a 

 maturation spindle are found before the time of ovulation in this species. 

 Since the study did not include the rupture of the ovarian wall by the oocyte, 

 definite cytological information concerning spindle formation is not avail- 

 able. King (1905), however, described the formation of the first maturation 

 spindle in Bufo as synchronous with the rupture of the wall of the ovary 

 by the oocyte, and the first polar body as given off during the passage of the 

 oocyte down the oviduct. The chief emphasis of this study of the mature 

 ovaries of specimens of Acris is not, therefore, on the behavior of the chromo- 

 somes, but on the seasonal changes in the morphological features of the 

 ovaries. 



