45° 



DISCOVERY REPORTS 



number of old corpora lutea lies in the uncertainty of the number which are formed 

 each season. Several years at least, and possibly many years, must elapse before the 

 last traces of a corpus luteum disappear, but as one cannot be sure that they do not last 

 indefinitely, any estimations of actual age must be applied only to the younger whales. 



Fin Whales 



* In which the total number of functional and old corpora lutea could be counted. 



In a polyoestrous animal the possible number of ovulations in any one season is 

 restricted by the supervention of pregnancy or by the season itself. It is probable that 

 the number will not be very great. Further, in a social and migratory animal like the 

 whale, one might hope to find, at least for a season or two after the attainment of sexual 

 maturity, a similarity in the experiences of the majority in respect of the number of 

 ovulations which do occur. For instance, all whales in their first year of sexual maturity 

 ovulate. After their first sexual season the minimum number of corpora lutea will be 

 one (the whale having become pregnant at the first ovulation) and the maximum 

 number will represent the number of dioestrous cycles, probably not much more than 

 half a dozen. A majority of whales would perhaps have ovulated the same number of 

 times, perhaps four or five corpora lutea being formed, and become pregnant. At the 

 next season these whales will begin lactation and no corpora lutea will be formed, but 

 further ovulations and a fresh batch of corpora lutea will occur at the third season. 



Assuming that the number of old corpora lutea is normally increased in this way in 

 alternate years, it is interesting to examine the frequencies of the numbers in which 

 they are found to occur in the ovaries. 



In Figs. 156 and 157 the frequencies of the numbers of corpora lutea are plotted for 

 Blue and Fin whales. These show that four to five and ten are the numbers of corpora 

 lutea which have been found most commonly in Blue whales' ovaries, and about four 

 to five, twelve, and possibly nineteen among Fin whales. 



The following explanation may be suggested for the prevalence of these numbers. 

 Taking Blue whales first we may call those with from one to eight corpora lutea Group i . 

 The whales in this group would be expected to consist mostly of those in their first 

 year after sexual maturity which have not become pregnant, those pregnant for the first 

 time, and those lactating or resting after lactation for the first time. Lactating whales 

 will of course be in their second year from sexual maturity. First pregnancies may be 

 recognized by the undeveloped state of the mammary glands. Any whales that are 

 pregnant yet show signs of previous pregnancy appearing in this group are three seasons 



