254 DISCOVERY REPORTS 



pairing mainly takes place in the months of May to August inclusive, with a maximum 

 in July, and also that a few pairings take place in other months outside the main 

 breeding season. The same curve also shows the frequencies of births, because the 

 period of gestation, as shown above, is approximately 12 months. Parturition and 

 pairing are thus taking place on the same grounds at the same time, but of course, 

 amongst different whales, for the parturient whales will not be pairing until the 

 following season some twelve months later. 



According to Figs. 77 and 78 few births take place in southern waters, for parturition 

 does not commonly occur until May, taking a length between 4 and 5 m. as that of the 

 young at birth. If parturition thus occurs from about May to August, lactation appears 

 to be completed and the mammary gland to be completely involuted for the most part 

 by the end of January, which gives a mean period of suckling of about 5 months. 



There now remains for consideration the large proportion of resting whales present 

 on the South Georgian whaUng grounds in the months of February, March and April. 

 The percentage of the total catch represented by resting whales increases steadily during 

 these months, showing that they remain behind later than the pregnant whales which 

 leave earlier, and may or may not have their numbers augmented by fresh arrivals. 

 Measurements of the mean diameter of the largest follicles in the ovaries of resting 

 whales are plotted by months as a scatter diagram in Fig. 79. From this and from 

 Fig. 80, where these data are summarized as the average mean diameter for each month, 

 it is seen that the size of the largest folHcle in the ovaries of resting whales is on the 

 whole increasing steadily through the season. The conclusion to be drawn is that these 

 whales are mostly maturing follicles which will be discharged later on in the season 

 in more northern waters to give rise to the pregnancies of the following season. Further, 

 the presence of the two distinct classes of whales, pregnant and resting, on the southern 

 whaling grounds in the earlier months of the year, is to be explained by concluding that 

 the female Sei whale breeds only once every two years and that anoestrus usually lasts 

 from the finish of lactation towards the end of the year until the second half of the 

 following year, after the northward migration. 



Referring now to Fig. 81, which shows the frequencies of the occurrences of various 

 numbers of corpora lutea in the ovaries, it will be seen that peaks occur for the numbers 

 I, 5, 8, II, 19, 23 and 27. If the number 14, which appears to be absent only because 

 of insufficient data, is interpolated, the peaks fall at distances of 3 to 5, which can be 

 interpreted as showing that at each oestrus from two to four dioestrous cycles, with 

 three to five ovulations, occur before pregnancy supervenes. Of the seventeen whales 

 having one corpus luteum, only nine had corpora lutea a. Eight of these were pregnant, 

 and the other appeared to have missed fertilization, because no foetus could be found 

 in spite of a very careful search. The remaining eight were all resting, and four at least 

 of these had been pregnant previously, as shown by the involuted state of the mammary 

 glands. No information is available about the mammary glands of the other four, but 

 in one of them there was a vaginal tag, showing that pairing at least had taken place, 

 though whether it resulted in pregnancy it is now impossible to state. At their first 



