They were of the same length ( 22.5 m. ) , but one possessed twenty-four old corpora lutea and a functional corpus luteum ( pseudo-pregnancy ) ; the other had one old corpus luteum and one of pregnancy .sx In the former , ankylosis of the epiphyses throughout the vertebral column showed that complete physical maturity had been reached ; in the latter , thick cartilaginous layers still separated the epiphyses from their centra .sx If the numbers of corpora lutea are plotted against the corresponding lengths of the whales , and the physically mature whales distinguished from those not mature ( Fig. 2 ) , it is readily seen that physical maturity bears little relation to length , but does bear a relation to the number of corpora lutea .sx Indeed , of 105 whales with less than fifteen corpora lutea only two , with eleven and twelve respectively , were physically mature ; while of 66 whales with more than fifteen corpora lutea only four , with sixteen , sixteen , twenty and twenty-one , were physically immature .sx The age at physical maturity is thus the number of years occupied in the accumulation of fifteen corpora lutea in the ovaries plus the two years that elapse between birth and sexual maturity .sx One of the applications of this result is that if the number of corpora lutea has been noted , comparisons can be made between the number of physically mature whales caught in different seasons and areas .sx This applies particularly to our own previous .sx work , but should be of value if any future observations are made from floating factories , where the genitalia can often be obtained when there is no opportunity of examining the vertebral column .sx When further work has been done on these lines the restriction of comparison to the physically mature whales of successive seasons should lead to a more accurate indication of increase or diminution of stock than is at present available .sx The sexual season is limited in some animals to a single oestrous period , and , ovulation taking place at oestrus only , the accumulation of corpora lutea will be slow or rapid according to whether one or more than one ovum is normally shed at this time .sx In other .sx animals the oestrous ( dioestrous ) cycle is shorter , and is repeated during the sexual season until pregnancy supervenes or the season ends .sx In these polyoestrous animals corpora lutea will accumulate rapidly .sx In some animals again the ovum is not shed unless coition occurs , that is to say , only corpora lutea of pregnancy will be found in the ovaries .sx It is obviously of the utmost importance to discover the condition normally existing among whales if there is to be any solid foundation beneath a theory involving the corpora lutea .sx The reasons for considering Blue and Fin whales as polyoestrous animals in which ovulation occurs regardless of coition are given in the Report ( 1929 ) on p. 390 .sx It was there shown that , although foetuses are sometimes lost at sea , and perhaps occasionally very early stages are missed in spite of thorough search , there are too many apparent pregnancies in which no foetus can be found to be accounted for in any way other than by ovulation .sx This view has received support from observations during 1929-30 .sx Thirteen apparent ovulations have been noted .sx The size and condition of the uterus in these whales did not suggest the presence of a foetus , although little reliance can be placed on measurements and observations of congested condition in an organ so capable of rapid recovery as the uterus .sx When a foetus has been present some indication of the occurrence can usually be found .sx During this season three foetuses are known to have been slipped at sea , and one of them , about 0.2 m. long ( No .sx 2753 ) , was discharged in its membranes from the parent whale upon the deck of the catcher that was towing it' .sx Whales have been recorded in which two or more ova have been liberated at the same time , but they are too few to form a serious exception to the rule of normal behaviour , which is the shedding of a single ovum at each ovulation .sx ( See note on Multiple Ovulations on p. 419 .sx ) That ovulation takes place without the stimulus of coition is now almost beyond doubt .sx The high percentage of ovulations at South Africa ( Saldanha Bay , 1926 ) and the comparatively large number of ovulations recorded from South Georgia suggest this condition .sx The only direct evidence is the capture this season of a Fin female with .sx one old corpus luteum in the ovaries , immature mammary glands , and an unbroken vaginal band ( No .sx 3070) .sx A similar whale was reported in 1925 .sx Fin and Blue whales are thus polyoestrous animals in which ovulation is spontaneous , one ovum is discharged at a time , and all ovulations leave a permanent record in the ovaries .sx The question now arises whether something cannot be done towards the estimation of the actual age from the number of corpora lutea .sx It has been noticed this season , as in previous seasons , that certain numbers of corpora lutea appear to be much more frequent than other numbers .sx An explanation of this fact was attempted in 1929 , and the theory of age , fully explained below , was briefly outlined at the same time ( p. 450) .sx In polyoestrous animals with a definite sexual season , the number of ovulations in any one season depends upon the occurrence of pregnancy within the limits imposed by the season itself .sx In .sx social animals like whales , that live in communities or schools and undertake extensive migrations more or less simultaneously at certain times , one would expect to find , at least for one or two seasons after sexual maturity , that the experience of all had been somewhat similar , and that circumstances or conditions that had affected one had affected others in the same way .sx This certainly applies to the course of migration .sx There would be no " Fin years " or " Blue years " at South Georgia ( seasons when one species appears in great numbers while the other is practically absent ) if conditions affecting migration did not affect the whole herd .sx Take then , for example , a group of whales in their first sexual season .sx At the close of the season the minimum number of corpora lutea will be one ( corpus luteum of pregnancy ) and the whale will be pregnant , the maximum will represent the number of dioestrous cycles that are possible during the season .sx If , in the majority of these whales , fertilization of the ovum for some reason does not occur for one or more oestrous periods , there will be a majority of whales haying the same number of corpora lutea in the ovaries ; that is , there will be traces of a certain number of ovulations and a corpus luteum of pregnancy .sx In the following season these whales will be lactating , while a fresh group is becoming mature .sx This fresh group may undergo the same experience as the previous whales or it may not , but the number of corpora lutea cannot exceed the maximum which is limited by the season .sx As gestation and lactation occupy most of two years , it is during their third season that the first whales again experience a sexual season and the number of corpora lutea again increases .sx The second group is now lactating , so the number of corpora lutea remains stationary , and a third group is verging upon maturity .sx In the following year the first group is again lactating , the second ovulating , the third lactating , and a fourth is becoming mature .sx Thus , in representative samples from one area , the frequency of the numbers of corpora lutea will show a succession of nodes or peaks marking the increase in numbers every two years .sx The graph of the frequency of corpora lutea for season 1929-30 at South Georgia is shown in Fig. 3 .sx Peaks at one , seven , and eleven are clear , and there are indications .sx of others at eighteen and twenty-one .sx The whales can be grouped round each of these peaks as follows :sx Group I , whales with one , two , three or four corpora lutea ; Group II , whales with five to nine ; Group III , with ten to fourteen ; Group IV , with fifteen to nineteen ; Group V , with twenty to twenty-four .sx We have thus in the number of corpora lutea an indication of the age of any female Fin whale up to eight years from sexual maturity ; and because each group is composed mainly of pregnant and lactating whales or of resting whales one year older , we can obtain a rough idea of the number of each age caught during the season from the number of whales in each group as indicated by the frequency chart .sx The assumption has here been made that pregnancy recurs every two years .sx That this is in all probability normal behaviour is shown in the Report ( 1929 , p. 431 ) from consideration of the percentage of pregnant and resting whales .sx During 1929-30 pregnant , lactating and resting whales constituted 64 , 17 and 12 per cent respectively of the catch of mature females .sx The remaining 7 per cent were ovulating .sx The percentage of pregnant whales is rather high theoretically it should be 50 per cent but allowing for possible segregation and seasonal variation the figures point to a recurrent pregnancy at intervals of two years .sx In several resting whales the size and condition of the mammary glands showed that lactation had not long ceased , and seven whales from a total of 199 were pregnant .sx and lactating at the same time .sx In one lactating whale ovulation without subsequent fertilization had occurred .sx These pregnant and lactating , or ovulating and lactating whales suggest either that lactation is sometimes unduly prolonged , or that postpartum ovulation may sometimes take place .sx If the latter is the true explanation , the interval between successive births in these whales is one year instead of two .sx In Fig. 4 the frequency graphs for six seasons at South Georgia can be compared .sx In the two previous " Fin years " ( 1925-6 ; 1928-9 ) a grouping similar to that of 1929-30 can be recognized , although the peaks alter their position within the groups according to the number of unsuccessful ovulations that was most frequent in that particular season .sx It will be noticed in the previous work on the frequency of the corpora lutea ( Report , 1929 , p. 451 ) that Group I included the present Groups I and II .sx The present Group I with a maximum at one was not evident because , in two of the three seasons reported upon , the early part of the season was missed , and it can be shown by the incidence of pregnant whales with a single corpus luteum that the earliest pregnancies of the season arrive early at South Georgia .sx During this season there were eight single corpus luteum pregnancies in November ( thirty-five mature whales ) , four in December ( eighty-one mature whales ) , five in January ( seventy-six mature whales ) , one in February ( twenty-nine mature whales ) , and none in March .sx Last season there were four in October ( thirty-one mature ) , two in January ( forty-two mature ) , and one in February ( fifty mature) .sx From the single functional corpus luteum it is evident that these whales had been fertilized at the first ovulation of the season .sx There is a noticeable difference between Group I and all the subsequent groups in that the greatest frequency occurs at the beginning rather than near the middle ; in other words , that in the first sexual season the majority of whales become pregnant at the first ovulation , while in subsequent seasons unsuccessful ovulations usually precede pregnancy .sx A possible explanation is that whales nearing the time of maturity tend to stay near the breeding areas , or , at any rate , do not make a long southward migration , and they are thus the first to be impregnated , while the older whales arrive from the south somewhat later , after one or more ovulations not usually fertilized .sx In Fig. 5 the figures for 393 mature Fin females the total catch since 1924-5 are combined , and it can be seen that , notwithstanding the overlapping that must necessarily occur , the grouping is still evident .sx While it is not suggested that the groups in themselves give more than an indication of difference in age of a number of whales , some idea of the amount of overlap between the groups can be gained in the following way .sx