THE CORPUS LUTEUM 359 



Vesicular corpora lutea 



In Text-fig. 5, these morphological types have been arranged in the order of their development. They 

 fall into two main groups ; the non-vesicular types (c-g) which are obviously related, and the corpora 

 with cavities (types h-l). In a large sample of 701 corpora lutea which were grouped according to the 

 presence or absence of a cavity 17-1% had a cavity. Often the vesicular type lack a corona and the 

 stigma is inconspicuous. In the single specimen of type k, the thinner part of the wall gave the impres- 

 sion that it had been under pressure from the liquor folliculi. The connective tissue septa are radially 

 arranged in a regular pattern. This radial arrangement of trabeculae and internal septa is observed in 

 all the corpora with large vesicles, but not in type /. 



The mode of formation of the cavity calls for some comment since Dempsey and Wislocki (1941) 

 and Robins (1954) have suggested that cavity formation in the humpback whale is related to the large 

 size of the corpus luteum. Robins suggested that the absence of a cavity is diagnostic of a corpus 

 luteum of ovulation and this, if true, would be of great value in studies on the reproductive cycle and 

 of age-determination. 



Dempsey and Wislocki base their conclusions on the examination of 16 pairs of ovaries, not all of 

 which were from pregnant females. The corpora lutea they examined were 3-6 cm. in diameter so, 

 even allowing for excessive shrinkage as a result of fixation (which is not borne out by the histological 

 appearance), they are well below the normal size range for corpora lutea of pregnancy in both fin and 

 humpback whales. In fact it appears likely that they were corpora lutea of ovulation. These authors 

 ' conclude that the large size of the whale's corpus luteum imposes certain anatomical peculiarities 

 and physiological limitations on its vascular supply. The luteal arterioles and capillaries are apparently 

 unusually long. The functional restrictions in consequence of the lengthening of these vessels make 

 it impossible for the luteal tissue ever to fill the entire cavity of the luteal body. These limitations 

 result also in nearly complete avascularity of the centre of the corpus luteum, associated with excessive 

 fibrosis and hyalinization ' (p. 250). 



Robins (1954) found that in all the functional corpora lutea (37 in number) which he examined 

 from both early and late pregnancy there was a central cavity, 'variable in size but always quite 

 noticeable'. He also found that a variable proportion of corpora lutea [corpora albicantia?] did not 

 possess this characteristic cavity or core. Robins suggested that, owing to the small size of the corpus 

 luteum of ovulation (up to 88 mm.) there appears to be no anatomical limitation on the blood supply 

 and luteal cells continue to fill the cavity so that no distinctive centre remains. This is supported by 

 several whales which had ovulated twice without successful fertilization and showed the regressing 

 corpora lutea to lack the characteristic centre of the corpus luteum of pregnancy. According to 

 Robins, if pregnancy intervenes the gland continues to expand and, in accordance with the ideas of 

 Dempsey and Wislocki, anatomical limitations prevent blood vessels from supplying the centre of 

 the gland so that a thick avascular hyaline zone forms, limiting the thickness of the luteal tissue and 

 causing the central cavity to be retained. 



There are several important objections to this hypothesis. It has been pointed out that the corpora 

 examined by Dempsey and Wislocki were only 3-6 cm. in diameter, but nevertheless contained central 

 cavities. The corpus luteum of ovulation is of this order of size. Personal examination of humpback- 

 whale ovaries from the Antarctic (unpublished) shows that the non-vesicular corpora lutea of pregnancy 

 are more common than the type with cavities. Chittleborough (1954) illustrates a 12-cm. functional 

 corpus luteum of late pregnancy which clearly has no central cavity. 



It has been established that in the fin whale only 17-1 % of the corpora lutea have a cavity. Corpora 

 lutea are found up to 16 cm. in diameter which are without cavities and a blue-whale corpus luteum 



