39© DISCOVERY REPORTS 



are based on the condition of the vertebral column in some 23 whales for which fuller records are 

 available, and enable the progress of fusion to be fixed more accurately. 



For example, if only lumbar and/or caudal vertebrae are observed and lumbar 10 is FJV then 

 lumbar 2 (eight in advance) is assumed to be unfused, which puts this whale in stage 2. On the other 

 hand, if lumbar 15 is UTC then caudal 4 is assumed to be unfused, thus altering the classification 

 from stage 1-2 to stage 1. 



Stages 1-3 are regarded as physically immature, stages 2 and 3 approaching maturity and stage 4 

 whales are physically mature. It should be noted that this represents the average progress of fusion 

 with completion of linear growth in the anterior thoracic region, but there are a few exceptional whales 

 in which fusion appears to be completed in the middle or posterior thoracic region. It should also be 

 pointed out that the determination of physical maturity is less easy and less certain than the identifica- 

 tion of immaturity because the latter is dependent on any one unfused epiphysis, whereas a whale 

 might be classed as stage 4, and therefore mature, although the posterior thoracics were still unfused. 

 However, the good correlation between number of corpora and the attainment of physical maturity 

 suggests that errors are actually small. 



Table 6. Mean lengths of area II females in relation to numbers of corpora 



In Text-fig. 25 a smoothed curve indicating the increase in the mean length at increasing corpora 

 numbers is shown. This is based on 1233 sexually mature females taken in the antarctic area II, that 

 is, presumably from the same stock but over a period of years (Table 6). The mean length at physical 

 maturity in this sample is 73-0 ft. Purves and Mountford (1959) with a smaller sample from area I 

 obtained a figure of 72-9 ft. which is in close agreement. 1 As they point out, their sample may contain 

 a mixture of populations (Brown, 1954), and the present sample is composite in respect of time. 



For comparison with this curve of relative linear growth the frequency distributions of corpora 

 numbers at different stages in the progress of epiphysial fusion are also shown in this figure. The 

 ' immature ' group includes stage 1 and 1-2 ; the group ' approaching maturity ' is composed of stages 2 

 and 3, and the physically mature group is stage 4. 



The 'immature' group in which, it will be remembered, fusion has not progressed further than 

 the caudal region, corresponds to the period of fastest growth. The average growth is from 66-9 ft. 

 at one corpus luteum or corpus albicans to 72-5 ft. at the mid-point (8-5 corpora) where the ' immature ' 

 group overlaps the group 'approaching maturity'. The average amount of subsequent growth to 

 maturity is only 0-5 ft. and is mainly confined to the thoracic region. 



In Table 5 and Text-fig. 26 the smoothed frequency distributions of corpora numbers for physically 

 immature (stages 1-3) and physically mature (stage 4) females are drawn. These curves intersect at 

 14-3 corpora. 



1 Their value for a small area II sample is much higher. 



