4i6 DISCOVERY REPORTS 



whales; and, in the second place, the determination of the proportion of mature and 

 immature whales is a necessary preliminary to the estimation of various other ratios, 

 such as the percentage of adult females pregnant or lactating. 



The amount of data collected during the work makes it possible to fix with con- 

 siderable accuracy the mean length at which Blue and Fin whales become mature, 

 and, among all the whales examined, there have been very few cases in which maturity or 

 immaturity cannot be determined with confidence. The only doubtful cases are those 

 of whales whose length, being near to that at which maturity takes place, gives no clue, 

 and in which there are not sufficiently definite records of the condition of the genitalia. 



It is easy to determine whether a female is mature or not, but there is much more 

 difficulty in the case of males. 



Among females, information can be obtained from the following points: 



1. Presence of a foetus. This, of course, determines maturity, but it is of little value 

 in itself for estimating the mean length at which members of a species become 

 mature. 



2. Presence of corpora lutea in the ovary. This is the most valuable means of ascertain- 

 ing whether a female is mature or not. If corpora lutea (including the scars of very old 

 ones) are present in the ovary, the whale must, of course, be mature, and it can be said 

 that, with negligible exceptions, a whale without any sign of corpora lutea in the ovaries 

 is immature. As was 'explained in the section on the reproductive organs, it appears 

 that the old corpora lutea persist for several years, so that once ovulation has taken place 

 there will always be corpora lutea, or traces of them, in the ovaries. Instances of whales 

 which have just become mature without having yet ovulated are extremely rare, and it 

 might indeed be argued that a female need not be regarded as mature until it has actually 

 ovulated. In no case has a whale been examined which for other reasons was obviously 

 mature, but which had no traces of corpora lutea in the ovaries. 



3. Size of the uterus. There is, of course, an increase in the size of the uterus when 

 maturity is reached but it is not sufficiently sudden to constitute an infallible distinction 

 between the mature and immature. Among Fin whales an immature individual would 

 very rarely have a uterus measuring more than ii-ocm. across the cornu (i.e. the 

 transverse diameter of the collapsed cornu), and the uterus of mature whales would 

 rarely measure less. Among Blue whales the corresponding figure would be about 

 I2-0 cm. (see Figs. 131 and 132). 



4. Size of the ovaries. In a number of cases a figure has been used representing 

 roughly the volume of the ovaries and obtained by multiplying together the length, 

 breadth and depth expressed in centimetres. Among Fin whales a figure exceeding 

 about 800 generally indicates a mature whale. Among Blue whales a corresponding 

 figure would be about 900, but this is less certain. 



5. Weight of the ovaries. This is more convenient than measuring the size of the 

 ovaries. As stated in the section dealing with the reproductive organs, among Fin 



