388 DISCOVERY REPORTS 



other, and as regards humpback whales to the divergence of the results of Chittleborough (1955 6) and 

 Symons and Weston (1958). It is thought that the techniques used by the 'Discovery' Committee 

 and later by the National Institute of Oceanography have been fairly well standardized and that in 

 the present material individual differences in interpretation are reasonably small. 



There is, however, a basic cause of variation which must now be mentioned. Fusion of the epiphyses 

 to the centrum progresses from the centre to the periphery (Text-fig. 24) which means that the 

 determination of the state of fusion depends to some extent on the depth at which the line of fusion 

 is examined and is, therefore, influenced by the tools used to examine the vertebrae. When a knife 

 is used the cut is necessarily superficial; with a hand-axe or adze the cut is deeper, but more superficial 

 than the section exposed by use of a felling axe. Other things being equal, the more superficial the 

 examination, the later will be the apparent attainment of physical maturity. 



UTC 



UTC 



UFC 



FJV 



FJI 



Text-fig. 24. Diagram showing stages of epiphyseal fusion. Above, sagittal section of centrum and epiphysis; below, 

 appearance of tangential chips. The first example of FJI shows how the blood may indicate the line of fusion. 



Results 

 In analysing the data, certain simple conventions have been followed. 



(a) As regards the classification of individual vertebrae, fused join visible (FJV) is counted as fully 

 fused. Although the fact that the join is visible means that there is a thin layer of cartilage at the 

 periphery, this layer is not always continuous (e.g. Wheeler, 1930, PI. V, fig. 2) and usually there is com- 

 plete fusion in the less superficial parts (Text-fig. 24). Purves and Mountford (1959) have mistakenly 

 assumed this class to be unfused, but have used the figure of 14-15 corpora at the threshold of physical 

 maturity (which is dependent on the FJV classification being classed as fused). Wheeler (1930, 

 pp. 407, 408) also counted FJV as fused. If the FJV group is classed as unfused it naturally raises the 

 threshold of physical maturity in terms of corpora number, as the ovaries associated with Purves and 

 Mountford's ear-plugs show. 



(b) Five stages of fusion are recognized. (1) Some or all of caudal vertebrae unfused; lumbar and 

 thoracic vertebrae unfused. (1-2) Caudal vertebrae not seen; thoracic and lumbar vertebrae unfused. 

 (2) Caudal vertebrae fused; some or all lumbar vertebrae fused; thoracic vertebrae unfused. (3) 

 Caudal, lumbar and posterior thoracic vertebrae (11-15) fused; anterior thoracics unfused. (4) 

 Anterior thoracic vertebrae (3-5) fused. The material, classified into these stages, is set out in Table 5. 



(c) In classifying the observations in this way it has been assumed first, that if a vertebra is recorded 

 as FJV then the eighth vertebra forward of it will certainly be unfused, and secondly, that if a vertebra 

 is recorded as UTC then the fourth vertebra posterior to it will still be unfused. These assumptions 



