ACCUMULATION OF CORPORA UP TO ATTAINMENT OF PHYSICAL MATURITY 387 

 The body length at sexual maturity appears to be a fairly constant proportion of the body length at 

 physical maturity, and has been shown to average 85-1% (range 80-0-88-5, a = 3-14, V = 3-69) for 

 a number of cetacean species for which data were available (Laws, 19566). Recent work on pinnipeds 

 confirms that the age at sexual maturity in this group is related to the growth-rate, and it appears that 

 those species which are precocious sexually also show precocious physical maturity (Laws, 19566, 

 1959c). However, it seems unlikely that physical maturity is necessarily attained at a firmly fixed 

 number of years after sexual maturity, but rather that there is a similar spread, as in the age at puberty, 

 over a few years. 



Since Wheeler (1930) first demonstrated the correlation between physical maturity and corpora 

 number a large amount of extra material has been collected and it seems worth while briefly to re- 

 examine the relationship. 



Material and methods 



Wheeler (1930) had data on physical maturity and corpora number from 171 sexually mature female 

 fin whales. In the present material there are 925 sexually mature females for which data on the 

 progress of physical maturity are available, including 642 physically immature females and 283 mature. 



This material was accumulated between 1929 and 1949 and I have not included more recent data 

 because the present sample is sufficient to demonstrate the closeness of the correlation. Further 

 material could hardly improve the correlation, but might even increase the variation by adding to the 

 human errors (a further six workers being involved). I am indebted to my colleague Dr R. H. Clarke, 

 who had classified and tabulated this material and who kindly placed his material and a preliminary 

 analysis of the data in my hands. 



Some explanation of the procedure followed is desirable. Ideally it would be useful to have for 

 each whale as complete data as possible over the whole length of the vertebral column, but for 

 practical reasons this is impossible. On floating factory ships and to a lesser degree at shore stations 

 there is only a very short period during the working up of the whale (from the time when the meat is 

 removed from the backbone until the vertebrae are sawn up and put into the cookers), when it is 

 possible to examine the vertebral column. 



Observers are, therefore, instructed to examine the crucial vertebrae first, namely the fourth and 

 fifth thoracics. If these are unfused they then try the posterior thoracics and if these are still unfused 

 and time permits, then a lumbar vertebra, and if necessary a caudal vertebra, is examined. The rib 

 sockets and chevron bones are useful guides to the different regions. If an epiphysis is unfused a note 

 is made as to whether the cartilage is thick or thin, and if fused, whether the join is visible or invisible. 

 The vertebra is then classed as one of four categories according to the state of fusion—' unfused thick 

 cartilage ' (UTC), ' unfused thin cartilage ' (UFC), ' fused join visible ' (FJV) and ' fused join invisible ' 

 (FJI). These categories are illustrated in Text-figure 24. Because the epiphysis has a separate blood 

 supply from the centrum these two parts are sometimes differently coloured, either the epiphysis or 

 the centrum being engorged with blood. This means that the line of fusion can then be picked out 

 even in the fourth class (FJI). 



The observations on which this section is based have been made by no less than nine different 

 workers and this naturally results in an increase in the amount of variation. There are two main 

 sources of error: first, in the counts of ovarian corpora, some workers record ' doubtful ' corpora much 

 more frequently than others, and the ovaries on which the counts were made were sliced by hand, 

 a method which is likely to lead to small errors of omission. Secondly, and probably more important, 

 are the variations in individual techniques of examining the vertebral column. Attention has already 

 been drawn to the discrepancies between the results of the examination of fin whales by Wheeler 

 (1930) and Peters (1939) on the one hand, and Brinkmann (1948) and Nishiwaki (1950a, 1952) on the 



