2s8 DISCOVERY REPORTS 



this it is deduced that the period of most active growth is about i8 months, and that 

 a calf born at 4-5 m. in length in July becomes mature in the early part of the next 

 year but one, and breeds for the first time some 6 months later at the age of 2 years. 



Referring now to Fig. 81, in which the peaks suggest breeding seasons spaced at 

 intervals of 2 years, it is seen that the first peak appears to represent whales aged about 

 2 years, the second those aged 4 years, the third 6 years, and so on up to a maximum of 



14 years, so that it may be said, according to this interpretation, that a Sei whale aged 



15 years is an old one and that few captured exceed that age. 



The Sei whale, like other whales, reaches sexual maturity before growth is complete. 

 A number of observations on the state of physical maturity in Sei whales have been 

 collected and are presented in Table XXIII. Growth in length does not stop until 



Mdtj Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr Mdcj 



Fig. 84. Sei whale. Mean length of immature whales. 



all vertebral epiphyses are ankylosed to the centra of the vertebrae. The ankylosis starts 

 at each end of the vertebral column and proceeds centrally so that the middle and 

 anterior thoracic vertebrae are the last to continue growth. The stage of ankylosis 

 reached in several vertebrae in each whale is recorded in Table XXIII, which also 

 shows the approximate age of the females, as deduced from the number of corpora lutea 

 in the ovaries. The number of distinct ages of scars on the skin that were recognizable is 

 also given. 



Ankylosis of the epiphyses of the anterior caudal and lumbar vertebrae starts at an 

 age of about 4-5 years and progresses slowly towards the head. The process may be 

 complete by the age of lo-i i years, but in whale No. 3269 some of the thoracic epiphyses 

 were not fused at an age of 1 1-12 years. In the last female of the series aged 14-15 years 



