226 DISCOVERY REPORTS 



sexual maturity at fairly definite ages, but some variation may be caused by environment 

 and nutrition, and complete sexual maturity is not reached all at once. These points no 

 doubt apply also to whales, and our calculations must be concerned with the mean age 

 at which maturity is reached. For reasons given on p. 221 physical maturity is also 

 probably reached at a fairly definite age. This is when the epiphyses are fused with the 

 centra throughout the vertebral column. Fusion takes place at first at the ends of the 

 column, and the last vertebrae to be fused are the anterior thoracics (see Wheeler, 1930, 

 pp. 407-9). Since plenty of whales are found in the intermediate stages this progressive 

 fusion must take some time, and if the fusion is more advanced in one whale than in 

 another we can say that the former is likely to be the older, even though both are sexually 

 and neither is physically mature. 



The number of corpora lutea forms the most important criterion of age that has so far 

 been found. As explained above (p. 220) their value lies in the fact that they constitute 

 a more or less permanent record of the number of ovulations which have taken place. 

 The number of ovulations which can take place each year is limited, and since there is a 

 close correlation between the number of corpora lutea and the attainment of physical 

 maturity, these bodies come near to being a criterion of the absolute age of the whale. 



The significance of the old scars on the skin is described on pp. 214 and 215. They are 

 formed when the whale migrates to warmer waters, they appear to be permanent, and 

 whales with many corpora lutea generally have a correspondingly large number of scars. 

 It would scarcely be possible, however, to make reliable estimates of the numbers of 

 scars on whales, or, except in the youngest whales, to distinguish the separate age series. 

 Generally it can only be said that a whale with many scars is an old whale, or with few 

 a young whale. 



The most important problem is to ascertain how many corpora lutea on the average 

 are added each year in a sexually mature female. If this can be done the mean period 

 from sexual to physical maturity can be determined, and the average age of a number of 

 females in which the ovaries have been examined can be estimated. Wheeler's figures 

 (1930) indicated that physical maturity in Fin whales is reached when about fifteen 

 corpora lutea have accumulated in the ovaries. He found three peaks in a graph of 

 corpora lutea frequencies in physically immature whales, and inferred that Fin whales 

 become physically mature at from four to six years after sexual maturity. Laurie (1937) 

 found that physical maturity in Blue whales coincided with an accumulation of eleven 

 to twelve corpora lutea. His frequency curves did not suggest that any particular num- 

 bers of corpora lutea consistently occurred more often than others, but he considered 

 that certain of the old corpora lutea could be distinguished as of more recent origin than 

 the rest, and attributed them to ovulations which had taken place during the last breed- 

 ing season. From this he estimated that the average increment of corpora lutea after the 

 first adult season is slightly more than one per annum, and he claimed some corroboration 

 in a comparison of peaks in frequency curves for successive years. Wheeler and Laurie 

 would not, I think, claim any final certainty of their calculated rates of accumulation of 

 corpora lutea, and the estimations certainly need checking. 



