GENERAL CONSIDERATIONS 337 



Japanese workers presented a vast amount of data in a series of papers on the results of their 

 investigations (Nishiwaki and Hayashi, 1950; Nishiwaki and Oye, 1951; Mizue and Murata, 1951; 

 Ohno and Fujino, 1952; Kakuwa, Kawakami and Iguchi, 1953). These followed the methods 

 employed by Mackintosh and Wheeler (1929) and are in general agreement, though it is clear that no 

 consistent pattern is present in the occurrence of maxima in the frequencies of corpora. They give 

 average lengths at sexual and physical maturity, but there are variations and inconsistencies in their 

 findings. As criteria of physical maturity they accept fusion of the epiphyses in mid-lumbar or 

 thoracic regions and since fusion is in fact usually completed in the anterior thoracic vertebrae their 

 estimates are necessarily low. 



Ruud had shown that the ridges on the baleen plates could be used to estimate the ages of whales, 

 though after the first few years wear at the tips was greater than replacement in the gum so that the 

 method then gave minimum ages only (Ruud, 1940, 1945 ; Ruud and Jonsgard, 1950). The technique 

 is to record and amplify the variations in thickness of the baleen plate by means of a suitable apparatus. 

 In these recordings they claim to be able to distinguish annual steps or ridges associated with changes 

 in the nutritive state of the individual. As long as the pattern diagnostic of the baleen laid down in 

 lactation is present they feel they can be quite confident about the age. In his second paper, Ruud 

 showed that the age of the fin whale at sexual maturity was more likely 3 years than 2. In the third 

 paper it was shown that the blue whale also attained sexual maturity at later ages, averaging 5 years. 

 In the first season of maturity slightly less than two ovulations occur and corpora subsequently 

 accumulate at a rate of a little more than one a year; the maximum number of ovulations in any one 

 period of heat appeared to be four. Hylen, Jonsgard, Pike and Ruud (1955) published the results of 

 the examination of baleen plates from over seven thousand fin whales taken in the antarctic between 

 1945/46 and 1952/53. They state that they believe age groups O, I, II and III can be determined with 

 great exactitude, because traces of suckling baleen can be recognized. In older groups there is some 

 confusion, but they believe that the mistakes are few in group IV. Elsewhere (Hylen et al. unpublished 

 report) they stated that sexual maturity in the female is attained in groups II-IV, averaging 4 years of 

 age. A full account of the method is to be given in a later publication. 



Tomilin (1940, 1945) also drew attention to the use of the ridges on the baleen plates for ageing 

 whales and Nishiwaki in a series of papers (Nishiwaki and Hayashi, 1950; Nishiwaki, 1950a; 19506; 

 195 1 ; 1952) took up the problem. In the first two of these papers he drew attention to changes in the 

 coloration of the crystalline lens of the eye. He measured the absorption of light by the lens (expressed 

 as a percentage) using a photocell, and claimed that it increased regularly with age, as measured 

 relatively by length, ovarian corpora, testis weight and physical maturity. This is not a very con- 

 vincing piece of work, because large corrections are necessary to allow for the changes dependent on 

 variations in the time post-mortem. Nevertheless, Nishiwaki (1950 a) believed that it was more 

 accurate and reliable than any other method of age-determination although it does not give a measure 

 of absolute age. In the next two papers he takes up the question of ageing by means of the ridges in 

 the baleen plates by Ruud's method and using his recording apparatus. He suggests that the average 

 age at sexual maturity, which Ruud had placed at three years, should be four years, as Hylen et al. 

 (unpublished report) later confirmed. He also suggests that four corpora accumulate each baleen 

 period. Later (195 1) he examined the rate of growth in length of the baleen by comparing the distance 

 from gum to the first main ridge, with the length of baleen formed in the next period. This ratio he calls 

 the growth index, which is expressed as a percentage of the latter period, and he calculates the weekly 

 growth-rate during the whaling season. Assuming the growth-rate to be constant throughout the year 

 he found that the annual increment was equivalent to one full growth-period. In a later paper 

 Nishiwaki (1952) re-examined his data and combined the results of the work on the crystalline lens 



