338 DISCOVERY REPORTS 



with the baleen readings. He confirmed that the female fin whale attains sexual maturity at 4 years of 

 age and the male at t,\ years and used two large blue-whale foetuses to obtain an estimate of the degree 

 of coloration of the lens at birth. He then calculated the annual increment in coloration of the lens 

 from birth to sexual maturity (taking 4 years as the age at maturity) and applied this to the increase 

 in coloration from sexual maturity to physical maturity to obtain an estimate of the length of this 

 period, which he found to be approximately 6 years. He then assumes that two ovulations occur at 

 sexual maturity and in the fin whale a further 9-5 up to physical maturity. Then the annual increment 

 of corpora after the first year should be 1-5. The discrepancy between Nishiwaki's estimate of the 

 number of corpora accumulated at physical maturity (11-5) and those of earlier workers (13-15) 

 results from the use of different criteria as mentioned below, but this would not affect the result 

 since it applies to both number of corpora and coloration of the lens. In the same way he obtains 

 for the blue whale an average increment of corpora of 1-64 per year, and gives a table showing growth 

 in length from sexual maturity up to 12 years. Although the figure of two to three ovulations per 

 breeding cycle obtained by Nishiwaki agrees quite well with the conclusions put forward in the present 

 paper, there are serious objections to his methods. The most important is the controversial nature of 

 the evidence from the crystalline lens. Corrections are necessary to allow for the post-mortem changes, 

 and the estimate of the degree of coloration at birth seems to be little more than a guess. Recent 

 work both on baleen plates and on ear-plugs suggests that sexual maturity in the female fin whale is not 

 attained until on average 5 years of age. 



Since Mackintosh and Wheeler (1929) and Ommanney (1932) gave a general account of the repro- 

 ductive tract, this aspect of the reproductive biology has received little attention, apart from an 

 important paper by Matthews (1948). He demonstrated a well-marked cycle of change in the uterine 

 mucosa from sexual immaturity, pregnancy, lactation, and anoestrus correlated with the state of the 

 ovaries, mammary gland, etc. Post-partum involution is rapid and appears to occur without loss of the 

 mucosa. Slijper (1949, 1956) also gives some information on the reproductive organs during pregnancy. 



Relevant work on other species will be discussed later in this paper, but mention must be made here 

 of a valuable series of papers on the humpback whale (Megaptera novaeangliae) by Chittleborough 

 (1954, 1955a, 19556, 1958) who was able to work on this species in the southern winter near its 

 breeding grounds off West Australia. 



When the present work began in 1954 the general outline of the biology of the fin whale had been 

 established relatively unchanged since 1930. The female was assumed to be polyoestrous and various 

 estimates of the rate of ovulation had been made, none of which could be accepted without reservations. 

 No conclusive evidence of the complete persistence of corpora albicantia had been put forward and 

 they could not be used as measures of age except in a very general way. The estimates of age based on 

 baleen plates still required a final proof and could only be applied to young animals. The nature of the 

 evidence from the crystalline lens was controversial. 



Since then a new method of age-determination has been discovered by Purves (1955) and its value 

 confirmed by Laws and Purves (1956), Nishiwaki (1957), Nishiwaki, Ichihara and Osumi (1958) and 

 Purves and Mountford (1959). This depends on the presence of well-defined laminations in the 

 ear-plug of whalebone whales which, once laid down, constitute a permanent record. Work on large 

 collections of ear-plugs from antarctic fin whales is proceeding. This appears to confirm the figure for 

 the annual increment of corpora albicantia given in this paper, but suggests that the ages at sexual 

 maturity of both sexes are higher than previous workers have claimed. 



The major part of the present paper concerns the female reproductive biology, and the work was 

 carried out before the value of the ear-plug was known. Probably the most important confirmation of 

 the value of the ear-plug for age-determination lies in the correlation which has been found between 



