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NORTH AMERICAN FAUNA 74 



phase" of development; each had a fluid-filled antrum 0.5 mm in diameter or 

 larger (Harrison and Weir 1977). Follicles in the "first phase," before develop- 

 ment of the antrum, were seen only in histological preparations. Since follicles 

 with antra up to 5 mm in diameter occurred commonly in the ovaries of 

 immature females and of anestrus and pregnant adults, in any season, I assumed 

 that this was a resting stage, not related to proestrus. When the antrum had 

 increased in size to more than 10 mm in diameter, I regarded the follicle as ripe 

 or in the process of rapid development from the resting stage to the level required 

 for ovulation. 



Degenerate follicles were those containing milky, caseous, or calcified 

 material and not associated with an ovulation scar in the adjacent tunic. 



The corpus lutetnn of pregnancy in the first trimester was recognized by its 

 moderate size, about the same as that of a ripe follicle. It was pinkish to 

 yellowish and had a distinct ovulation scar in the adjacent tunic. In longitudinal 

 section, its outline was circular to slightly elliptical, with an irregular border 

 among the thecal folds. In the second to fourth trimesters, it was similar in 

 appearance to the foregoing but about twice as large, more elliptical, having a 

 smoother perimeter, and being highly vascularized. In the fifth trimester, at or 

 near full-term, it was slightly smaller than at the fourth trimester, pale reddish to 

 orange-brown, and contained up to 50% fibrous tissue, usually radiating from a 

 firm central core. 



The corpus luteum of pseudopregnancy . resulting from release of an ovum 

 that apparently was not fertilized, resembled the corpus luteum of pregnancy, 

 except that it was no more than half as large and showed retrogressive signs that 

 were greatly out of phase with the corpora in the ovaries of pregnant females. 

 Corpora that were not associated with an adjacent ovulation scar, probably 

 having retained the ovum, were of two types: accessory corpora lutea which 

 occurred in synchrony with a corpus luteum of pregnancy, and corpora atretica 

 which occurred in the absence of or not in developmental synchrony with a 

 corpus luteum of pregnancy. 



Each type of corpus luteum eventually degenerates, shrinking to a firm, 

 fibrous scar, the corpus albicans. The scar of the corpus luteum of pregnancy is 

 retained for several years, although it continues to diminish in size. 



I considered any female whose ovaries contained at least one ripe follicle, 

 corpus luteum, or corpus albicans to he fertile, i.e., potentially able to ovulate or 

 having already ovulated at least once. A female that had completed at least one 

 pregnancy was considered to he fecund. Any young animal that was not fertile 

 but had 10 or more vesicular follicles 2 to 6 mm in diameter in each ovary was 

 considered as having reached puberty; young fertile females that had not yet 

 completed a pregnancy and were physically immature were considered to be 

 adolescent. Physically and reproductively mature fecund females were regarded 

 as adults. 



Reproductive Organs 



Male 



The gross anatomy of the reproductive organs in a very young male walrus 

 was described by Murie (1871). The testes of adults are more oblong than those 



