CORPORA ALBICANTIA, CORPORA ABERRANTIA AND CORPORA ATRETICA 385 



appears to be related to changes in the thickness of the walls of the blood vessels. The relative thickness 

 of the vessel walls may be a better quantitative index of the age of a corpus than is its diameter. Since 

 the initial diameter of the corpus luteum of ovulation is only about two-thirds that of the corpus 

 luteum of pregnancy, it is likely that for a given vessel-wall thickness the ratio of the percentage thick- 

 ness of the vessel wall to the diameter of the corpus will be higher for those corpora albicantia derived 

 from corpora lutea of pregnancy than for corpora lutea of ovulation and could perhaps be used to 

 differentiate them. This is the only distinctive character which appears likely. 



There are two other types of corpora which are found in fin-whale ovaries. These are the corpora 

 aberrantia and corpora atretica. The former comprise 1-4% of all corpora albicantia and aberrantia 

 and appear to develop invariably from ruptured follicles. They are more heavily pigmented than 

 corpora albicantia and are also characterized by the absence of blood vessels from the lobes of ' luteal ' 

 tissue. Their frequency in different phases of the 2-year breeding cycle suggests that there is an ovulatory 

 period at the end of lactation or the beginning of the 'resting' period when they are formed. These 

 corpora are included in the routine counts of corpora albicantia for purposes of age-determination. 

 The corpora atretica are formed from unruptured atretic follicles and are easily distinguished from 

 true corpora lutea, albicantia or aberrantia. They are not included in the routine counts of corpora. 



We have now dealt with the ovaries and the formation and subsequent history of the corpus luteum. 

 It has been established that the corpora albicantia persist throughout the life of the female and from 

 the numbers at the different stages of regression it appears that they accumulate at the rate of approxi- 

 mately 1-4-1-5 per year although the possibility of 0-7-0-75 per year cannot be excluded. 



In the next part of this paper the accumulation of corpora albicantia up to the attainment of physical 

 maturity will be discussed. Then the annual cycle and life-history of the fin whale will be described 

 and the probable time and rate of ovulation will be discussed with a view to obtaining a second inde- 

 pendent estimate of the annual increment of corpora and also of the range of variation. 



ACCUMULATION OF CORPORA UP TO THE ATTAINMENT 



OF PHYSICAL MATURITY 



The evidence presented in the preceding section suggests that the average rate of ovulation in a stock 

 of whales does not vary appreciably from year to year or in different areas, but little has been said 

 about the amount of variation shown by individual females. An examination of the accumulation of 

 corpora up to a fixed point in the life-cycle, the attainment of physical maturity, is of relevance to this 

 problem and gives us some additional evidence. 



The frequency of corpora in early baleen groups 



It is shown below (p. 41 1) that, on average, females ovulate 1-42 times before becoming pregnant for 

 the first time. In studying the amount of individual variation in the rate of accumulation of corpora, 

 up to what is presumably a fairly regular age, the initial variation in the number of corpora in early 

 age-groups, caused by the spread of ages at puberty, must be taken into account. 



In the fin whale the evidence suggests that the majority of females ovulate for the first time at ages 

 ranging from 3 to 8 years (see below, p. 407). Thus an 8-year-old female may be primiparous with one 

 corpus luteum in the ovaries or have been mature for several years, with a correspondingly greater 

 number of corpora in the ovaries. In any one age-group there can, therefore, be quite a wide range 

 of corpora frequencies depending on the individual ages at puberty. 



Hylen et al. (1955) give the frequencies of numbers of corpora in the ovaries of sexually mature 

 female fin whales in baleen age-groups II-V. A description of this method of ageing may be seen in 



papers by Ruud (1940, 1945) and Ruud and Jonsgard (1950). 



8-2 



