348 DISCOVERY REPORTS 



follicle size is 278 ±0-15 cm. and the recently ovulated class of females has a mean maximum follicle 

 size of 3 -8 ±0-53 cm. This statistically significant difference disposes of the possibility that the 

 ' recently ovulated ' females were pregnant females in which a foetus had been missed in the search, 

 or even pregnant females which had aborted their foetus when harpooned. It is, however, possible 

 that very small embryos could have been overlooked in some of them. The occurrence of recently 

 ovulated females in summer in the Antarctic is a fact which is of great importance in establishing the 

 nature of the annual reproductive cycle in the female fin whale (p. 436). 



Chittleborough (1954) does not give figures for the size of the largest unruptured follicle in the 

 ovaries of humpback females after ovulation. He gives (in his fig. 3) maximum follicle sizes for females 

 approaching ovulation, which by his definition are females in which the largest follicle is over 3 cm. 

 in diameter. For 38 females with follicles over this critical diameter the mean maximum follicle size 

 is 4-4 cm., and the largest follicle, at 10-5 cm., was close to the time of rupture. One pair of ovaries 

 had maturing follicles measuring 53, 37, 35, 34 and 30 mm. 



Pregnant females 



It has been pointed out that most of the females in this class which have been examined are in mid- 

 pregnancy (with foetuses 0-5-3-0 m. in length). The frequency distribution of the diameter of the 

 largest follicle in 341 pairs of pregnant ovaries is shown in Text-fig. 2, the mode being 2-5 cm. and 

 the mean 2-78 ±0-15 cm. This is in contrast to the condition in humpback whales in late pregnancy 

 in which the mean diameter of the largest follicle in 45 pairs of ovaries is 6-4 ±0-9 mm. (Chittle- 

 borough, 1954). There are very few records from fin whales in the last quarter of pregnancy and the 

 diameter of the largest follicle in lactating whales (1-93 ±0-34 cm.) although smaller than in pregnant 

 females is not as low as in late-pregnancy humpbacks. However, in the Antarctic the majority of 

 lactating fin whales taken are in late lactation (p. 446). 



The records from pregnant females can be subdivided according to the foetal lengths so as to give 

 an indication of the variation in follicle size during pregnancy. This has been done for 341 females and 

 the result is given in Text-fig. 4, which shows the range, mean and twice the standard error of the 

 mean for each foetal length group. In the 0-0-5 m - g rou P f° ur records from foetuses of length 4 cm. 

 or less have been removed to the ' recently ovulated ' class. Out of 23 records for the 4-6 m. length 

 group, two are of maximum follicle diameters of 7 and 8 cm. These are clearly separate from the 

 remaining records and if used they introduce a heavy weighting; they are thought to be abnormally 

 persistent large cystic follicles and in calculating the mean size they have been rejected. Only two 

 records of follicle size from females with foetuses over 5 m. are available; the largest follicles were 

 respectively 8 cm. (mentioned above) and 2 cm. in diameter. 



The mean maximum diameter decreases from 3 -8 ±0-53 cm. in recently ovulated females to 

 2-53 ±0-3 cm. in the first few months of pregnancy, then appears to increase to 3-12 ±0-28 cm. at 

 mid-pregnancy followed by a decrease to 2-07 ±0-42 cm. about 1 month before parturition. With the 

 exception of recently ovulated females the values for the mean ± 2 s.E. overlap, owing to the variability 

 in follicle size, but the maximum values and the percentage of records over 3 cm. (Text-fig. 4) also 

 follow this pattern, so it probably represents a real cycle in follicle activity, with a period of about 

 5-6 months. Such follicular cycles are known during pregnancy in other mammals and are usually 

 of the same period as the dioestrous cycle (see p. 352). Usually the follicles enlarge but enter cystic 

 atresia before maturing, but in some species ovulation takes place with the production of accessory 

 corpora lutea. It is unfortunate that there is no material from late pregnancy for the fin whale, but it 

 is impossible to ignore the evidence concerning follicle size in late-pregnancy humpback whales, 

 because in other respects the two species have so many features in common. The values for late- 



