278 DISCOVERY REPORTS 



does the protozoan Haematophagus. Diatom film was very rarely noted, but scrapings 

 from the skin showed the frequent presence of several species of diatom and their 

 spores in small numbers. Like other whales the species is subject in the warmer 

 seas to attack by an unknown agent which cuts out ovoid pieces of blubber, leaving 

 wounds which slowly heal. 



A considerable list of internal parasites is recorded. Intestinal parasites were almost 

 invariably present in the Sei whale, the commonest being two species of cestode. 



The external and internal genitalia of both sexes are described. Histological material 

 from the testis was examined in twenty-four males and the results indicate that there 

 is no sexual season in the male, and that some males are fertile at all times of the year. 



Dated records of foetal lengths show that the period of gestation is about 12 months, 

 and that the young are born at a length of about 4-5 m. Lactation lasts about five months 

 and weaning takes place when the calf is 8-9 m. in length. 



The main part of the pairing season occurs from May to August with its peak in 

 July. As the period of gestation is about 12 months the season of parturition is the 

 same. A few pregnancies, however, start in other months of the year, showing that 

 oestrus may occur outside the main breeding season, perhaps sometimes owing to the 

 loss of calves. 



The results of a detailed examination of the internal genitalia of a full series of Sei 

 whales indicate that the species breeds once every two years, with a period of anoestrus 

 of about 6 to 7 months between the end of lactation and the beginning of the next 

 pregnancy. 



The numbers of old corpora lutea in the ovaries indicate a polyoestrous sexual cycle, 

 with an average of three dioestrous cycles of four ovulations at each cycle succeeding 

 the first. 



Sexual maturity is reached at an age of about 18 months, and breeding first occurs 

 at the end of the second year after the birth of the whale. Fusion of the epiphyses of 

 the vertebrae starts at an age of 4 to 5 years and is usually complete at an age of 10 to 

 1 1 years, when physical maturity is reached. The oldest whale of the present series 

 was aged about 15 years. 



Logarithmic plotting of the mean values of body measurements against total length 

 of whale does not show the marked concentration of growth in the head region that is 

 found in other species. The relative growth rates of various parts of the body show 

 on the whole a simple relationship to the total length, but the growth is much more 

 evenly distributed throughout the body than in some other species. The absence of 

 the disproportionate growth of the anterior end, seen in larger whales, is probably 

 correlated with the small size of the species and the lower food requirement. 



The migrations of the Sei whale consist of a feeding migration to the south in the 

 southern summer and a breeding migration towards the north in the winter. Parturition 

 and pairing occur mainly in tropical and sub-tropical waters. Lactation is mainly 

 finished by the time the whales arrive on their southern feeding grounds. Pregnant 

 whales are the first to arrive at and leave the southern grounds, and are followed by 



