220 DISCOVERY REPORTS 



most important are the corpora lutea which not only indicate the sexual condition of the 



whale, but reveal much of its sexual history and age. 



PERSISTENCE OF THE CORPORA LUTEA OF THE OVARIES 

 Investigation of the growth and age of whales relies to a large extent on the per- 

 sistence of old corpora lutea as recognizable bodies in the ovaries, and it is perhaps worth 

 while to review the evidence on which this persistence is assumed. When ovulation takes 

 place the follicle from which the ovum is discharged becomes filled with luteal tissue and 

 forms a solid corpus luteum which in whales may measure 10 or more centimetres in 

 diameter. If impregnation takes place the corpus persists with little change as a func- 

 tional body throughout the period of gestation. On the birth of the calf it shrinks to a 

 much smaller, tougher body which is more properly described as the corpus albicans. 

 It has generally been found convenient to include it in the term ' corpus luteum ', and to 

 refer to 'functional' and 'old' corpora lutea, or sometimes to 'corpora lutea a and b\ 

 If ovulation is not followed by impregnation a functional corpus luteum is formed in the 

 same way but is presumed (by analogy with other mammals) to last for a comparatively 

 short period before shrinking to the smaller body. 



The number of these old corpora lutea to be found in a pair of ovaries of an adult 

 whale may vary from one to over fifty, and this fact alone suggests that they may be 

 cumulative. Whales have a definite breeding season, and since no mature female is ever 

 found (except those which are evidently only just verging on maturity) without any 

 corpora lutea, new or old, in the ovaries, it is evident that these bodies persist at least 

 from one breeding season to the next. Furthermore, if the number of corpora lutea in 

 the ovaries is plotted against the length of the whale it is found that the smaller whales 

 (which have not grown more than i or 1-5 m. beyond the mean length at which sexual 

 maturity is reached) always have comparatively few corpora lutea. The larger whales, 

 on the other hand, may have any numbers of corpora lutea, except that the largest of all 

 never have very few (see Mackintosh and Wheeler, 1929, p. 394). This is entirely con- 

 sistent with the steady accumulation of corpora lutea, for it is reasonable to suppose that 

 all whales continue to grow a little after reaching sexual maturity, and that some cease 

 growing at a certain point while others go on to reach greater lengths. The minimum 

 length at which growth ceases would be the point beyond which the numbers of corpora 

 lutea cease to be related to the lengths of the whales, and it may also be supposed that, 

 in the period required to attain the greatest lengths, a whale could not fail to accumulate 

 at least a moderate number of corpora lutea. The argument is clinched by a comparison 

 of the numbers of corpora lutea in the ovaries of sexually mature whales which have and 

 have not reached full physical maturity. A whale is said to be physically mature when 

 the epiphyses 1 of the vertebrae have become fused with the centra throughout the 

 vertebral column, and it is well known that such ankylosis in any mammal marks the 

 termination of linear growth. Wheeler (1930, p. 41 1) showed that in Fin whales physical 



1 The epiphyses are the bony disks at each end of the centrum or main body of the vertebra. In the 

 unfused state they are separated from the centrum by a layer of cartilage. 



