SUMMARY 479 



maturity, have been compared. These frequency curves are found to be very similar in shape and 

 range, with standard deviations of respectively 3-3 and 3-94 corpora. This amount of variation is 

 almost entirely explained by the spread in the ages at puberty, and suggests that there is a very regular 

 annual increment of corpora, which is difficult to explain on the basis of a polyoestrous sexual cycle. 



10. Histological examination of a series of fin whale testes indicates that the period of maximum 

 activity extends from about April to July and is minimal in January and February. The seminiferous 

 tubules undergo a seasonal variation in diameter, decreasing progressively from 164// in October to 

 140 n in February and rising again to 165 // in April. The maximum diameter recorded (241 /i) was 

 from March. Reference to the incidence of diatom film (which is absent in lower latitudes), indicates 

 that on entering antarctic waters the tubule diameter is about 170 //, falling to 140 ft when the diatom 

 film is well developed. In immature males the tubule diameter averaged 79^ and in one near to 

 puberty 102 ft. 



11. A mean curve of foetal growth is given and from this the conception dates of individual 

 foetuses can be estimated. 12 June is estimated to be the mean date of pairing, but the frequency- 

 distribution of conceptions is skewed, with a peak in May /June. It is estimated that 77 % of foetuses are 

 conceived between April and August, and only about 6% between October and March. As the gesta- 

 tion period is about 1 1 ^ months the frequency distribution of births can easily be obtained. Annual 

 variations in the timing of the breeding season are discussed. 



12. Earlier estimates of the mean length of the female at puberty (65-25 ft.) have been confirmed 

 on the larger samples now available; the standard deviation of the length at puberty is 2-07 ft., but 

 this is influenced by the post-mortem history of individuals and the variance in life is probably much 

 less. The average age at puberty based on ear-plug ages and ovarian examination is estimated to be 

 about 5 years, ranging from 3 to 8 years in the relatively small sample available. 



13. The state of the mammary gland is used to diagnose nulliparous and primiparous females. Of 

 88 females pregnant for the first time 68 % had only one corpus luteum and only 9 % had more than 

 two corpora. The mean number of ovulations preceding conception was 1-42. 



The pairing season of primiparous females was estimated, like that for all females, from foetal 

 length data, and it is found that the median date of pairing is about 21 July. That is about 4-6 weeks 

 later than multiparous females, and the frequency curve of conceptions is symmetrical, not skewed. 



The rate of growth in length for females in their first pregnancy can be determined, employing 

 foetal age as a time-scale. The average female grows from 65-25 ft. to about 68-69 ft- during the twelve 

 months following puberty. There appears to be a marked increase in the growth-rate related to the 

 short annual period of intensive feeding. The growth in the following year has also been studied. 



14. Evidence is presented which suggests that ovulation is spontaneous. It appears that at puberty 

 there is a single ovulation, and if the female is not successfully mated she goes into anoestrus without 

 experiencing further ovulatory cycles at this time. Although puberty is normally attained in July, it 

 may also be attained outside the normal pairing season, mainly in November, December, and January. 

 Then also there is a monoestrous cycle, but it is almost invariably unsuccessful because, it is believed, 

 the males are sexually inactive at this time. 



The relation of puberty to the migratory cycle is discussed. In the majority of females puberty is 

 preceded by a period of intensive feeding in the Antarctic, when growth is probably rapid, but the 

 main exteroceptive factor bringing about the first ovulation appears to be the period of increasing day 

 lengths associated with the northward migration. There is a second, subsidiary period when other 

 females attain puberty in December, after a southward migration, during which they also experience 

 increasing day lengths. It is suggested that puberty is initiated by a combination of age and increasing 

 day lengths, the light threshold becoming lower with increasing age. 



