ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



189 



The earliest corpus luteum (14 February) was 25 mm in diameter and had a 

 prominent, incompletely healed ovulation scar in the adjacent tunic. About 

 three-fourths of the corpus was luteinized; the portion that was not luteinized 

 contained several fluid-filled cavities. Three unruptured follicles 8 to 12 mm in 

 diameter that were adjacent to it also were partly luteinized (accessory corpora). 

 In histological section, the lutein cells of the ruptured follicle were round to oval, 

 13 to 19 /xm in diameter, mostly with rather oval to oblong, darkly stained 

 nuclei, mostly with no distinct nucleoli. The cytoplasm appeared "frothy" with 

 secretion droplets and lacked vacuoles (Fig. 116a). 



Of 27 specimens obtained in March and April, 20 (74%) already had a firm, 

 well-formed corpus luteum. The other seven had follicles up to 9 mm in 

 diameter. The corpora lutea in each of the 20 specimens were rounded and 

 contained no fluid-filled spaces or hemorrhage; they ranged in diameter from 17 

 to 28 mm. Histologically, a specimen taken on 17 March showed lutein cells 

 more rounded than those in the February specimen and slightly larger (17 to 25 

 /xm); the large nuclei were rounded to oval, mostly with a prominent, darkly 

 staining nucleolus (Fig. Il6b). The cytoplasm was frothy, with no vacuoles, and 

 there was a remarkably large amount of interstitial vascular development. The 

 corpus from one taken on 1 April had somewhat larger lutein cells, 17 to 33 /xm in 

 diameter. These differed from the foregoing in having numerous large vacuoles 

 up to 19 ^m in diameter and indistinct nucleoli (Fig. 116c). 



Of 154 specimens taken in May and June, 105 (68%) already had ovulated. At 

 least 10 of the others had ripe or ripening follicles larger than 10 mm in 

 diameter. The largest of those follicles (29 mm in diameter) was degenerate, as 

 indicated by its milky liquor folliculi. In the 105 specimens that already had 

 ovulated, the corpora lutea of pregnancy were well developed and grossly 

 resembled those from the March-April specimens, but they were mostly 

 somewhat larger (18 to 40 mm). Three of these were examined histologically. 

 The first, from an animal taken on 30 May with a 0.7-mm unattached blastocyst 

 in the uterus, had lutein cells that were larger (31 to 45 /xm) and more rounded 

 than in any of the foregoing. These cells contained an abundance of vacuoles up 

 to 20 /xm in diameter (Fig. 116c/). The nuclei also were large and rounded to 

 oval, with a prominent, usually peripheral nucleolus. The second corpus, from 

 an animal taken on 22 May with a 6-mm elliptical blastocyst, was similar to the 

 first, except that the lutein cells were slightly larger (33 to 48 /xm). In the third 

 corpus, from an animal taken on 10 June with a newly implanted embryo, the 

 cytoplasm of the lutein cells was somewhat finer-grained, and the nucleoli were 

 more often central than peripheral (Fig. 116c). The lutein cells in this corpus 

 were larger than those in any of the foregoing (37 to 58 /xm in diameter). 



Of 23 specimens taken in July and August, 14 (61 % ) had a large corpus luteum 

 of pregnancy; a few had corpora atretica, and 2 had ripe follicles up to 13 and 19 

 mm in diameter. I did not examine the corpora lutea of any of these 

 histologically. 



The corpora lutea of specimens taken during the autumn and winter tended to 

 be still larger (Fig. 115). Judging from histological examination of two of those 

 corpora from animals taken on 30 November and 8 February, there is gradual 

 fibrous infiltration of the corpus throughout the winter and some diminution in 

 size of the lutein cells. In the November specimen, the lutein cells ranged in 



