478 DISCOVERY REPORTS 



mature females. In the majority of immature females the largest follicle is less than i cm. in diameter, 

 but at the approach of puberty the mean maximum follicle size rises to about 3 cm. or more between 

 June and November. 



In mature females the mean maximum follicle size is greatest in the 'recently ovulated' class 

 (3 -8 ± 0-53 cm.), and is lower in pregnant, lactating, and ' resting ' females (respectively 278 ± o- 1 5 cm., 

 1 -93 ±0-34 cm. and 1-93+0-29 cm.). The existence of a cycle of follicular activity has been demon- 

 strated in pregnant females, reaching a peak in mid-pregnancy (November/December). At this time 

 the follicles enlarge, but do not mature, owing to the presence of the active corpus luteum. No acces- 

 sory corpora lutea form during pregnancy as happens in some other mammals. From the follicle 

 sizes it appears that there are periods of follicular activity in early lactation and at the end of lactation. 



6. Ovulation probably occurs when the follicle attains a diameter of about 7 cm., collapsing to 

 about 4 cm. immediately after ovulation and then, as the corpus luteum forms, increasing to 8-28 ± 

 0-82 cm. in the corpus luteum of the cycle, or to 11-44 + 0-15 cm. if pregnancy supervenes. There is 

 no reason to suppose that pseudopregnancy occurs. It is possible that there is a slight tendency for 

 the right ovary to ovulate more frequently than the left in baleen whales, but the data for blue and 

 fin whales do not show a significant difference. There is a marked tendency for ovulation to occur in 

 the vicinity of the anterior pole of the ovary. The mean weight of the corpus luteum of ovulation is 

 0-375 kg. (range 3-4 g.-i-5 kg.) and the mean weight of the corpus luteum of pregnancy is o-88i kg. 

 (range 0-2-2-4 kg-)- Various morphological types of corpora lutea are described and related to the 

 modes of formation; vesicular corpora lutea constitute 17-1% of all corpora lutea. Accessory corpora 

 lutea comprised only 3 -7 % of all corpora lutea examined ; they have a mean diameter of 3 -88 ± o-8o cm. 

 and a weight of about 45 g. The size ranges of corpora lutea of ovulation and pregnancy are large, so 

 that these bodies cannot be distinguished from one another on the basis of size, and no certain diag- 

 nostic features have been found. 



7. There is no significant difference in the mean size of corpora albicantia in the ovaries of pregnant, 

 lactating, and 'resting' females; they have a mean diameter of 2-5 cm. and a mean weight of about 

 10 g. The various morphological types of corpora lutea are reflected by the corpora albicantia and, 

 in particular, the incidence of vesicular or radiate corpora albicantia is not significantly different from 

 the proportion of vesicular corpora lutea. 



It appears that corpora lutea regress to a constant proportion of their initial size, and as this is large 

 they remain visible throughout life. 



Three age groups of corpora albicantia have been identified on the basis of anatomical and histo- 

 logical changes associated with regression. 'Young' corpora albicantia have a mean diameter of 

 4-01 cm. and a weight of 41 g. ; the values for 'medium' corpora albicantia are 2-94 cm. and 15 g., 

 and for 'old' corpora albicantia 2-01 cm. and 5 g. 



The ratio of ' young ' to ' medium ' corpora albicantia in the ovaries is 1 : 2 and it is shown that the 

 ' young ' corpora albicantia probably represent one year's increment of corpora and take about three 

 years to regress to 'old' corpora albicantia. From this it can be calculated that the mean annual 

 increment of corpora is probably about 1-4-1-5 (although there is a possibility that the rate is only half 

 this value), and as the corpora persist throughout life, counts of them may be used to determine age. 



8. Two other structures in the ovaries are described, the corpora aberrantia and the corpora 

 atretica. The former are included in the corpora counts (as corpora albicantia) for the purpose of age- 

 determination, but the latter are ignored. 



9. It has been possible to confirm earlier work on the correlation between the attainment of physical 

 maturity and the accumulation of 14-15 corpora lutea and corpora albicantia. The frequency distribu- 

 tion of the number of corpora shortly after puberty (baleen group v), and at the threshold of physical 



