4 i2 DISCOVERY REPORTS 



The monthly frequencies of conception for newly mature females can be estimated by relating the 

 individual foetal lengths to the mean curve of growth as was done previously for all pregnant females 

 (p. 403). This involves the assumption that the rate of foetal growth is the same in primiparous females 

 as in multiparous females; it is thought to be a reasonable premiss and there is no evidence to the 

 contrary. The conception periods estimated in this way for several groups of newly mature females 

 are shown in Text-fig. 37. The classes of newly mature females for which conception dates have been 

 calculated are as follows: (1) Primiparous females with one corpus luteum and up to three corpora 

 albicantia in the ovaries. Examination of the mammary glands is used to diagnose a first pregnancy 

 in this case. They will be referred to as ' primiparous ' females. (2) Females known to be primiparous 

 because they are pregnant with a corpus luteum but no corpus albicans in the ovaries, that is, they 

 conceived at the first ovulation. This group includes females for which there is no mammary gland 

 data; it will be referred to as the 'o corpus albicans' group. (3) Females probably in their first or 

 second pregnancy with a corpus luteum and one or two corpora albicantia ; to be referred to as the 

 ' 1 corpus albicans ' and ' 2 corpora albicantia ' groups. 



The frequency of conceptions of the ' primiparous ' group of (88) newly mature females is shown in 

 Text-fig. 37, centre (Pj. The frequency curve is fairly symmetrical, with mean, modal and median 

 dates all approximating to late July (respectively 22, 26 and 21 July). It will be remembered that the 

 conception curve for all females was markedly skewed towards earlier months. The combined con- 

 ception frequencies of the ' o corpus albicans ' and ' 1 corpus albicans ' groups has been plotted in 

 the same figure for comparison with the ' primiparous ' group. This is a larger sample (151) and shows 

 a closely similar frequency distribution to the 'primiparous' group, the mean, modal and median 

 dates being very close to these values for the 'primiparous' group (respectively 21, 26 and 19 July). 

 For the purpose of determining the monthly frequency of multiparous conceptions it will, therefore, 

 be assumed that the ' 0-1 corpora albicantia ' group is equivalent to all first pregnancies (below, p. 425). 

 This means that the samples of primiparous and multiparous females need not be restricted to those 

 females for which mammary gland data are available, but only to those for which the results of examina- 

 tion of the ovaries are available. The number of pregnant females for which foetal length records and 

 ovarian data are available is 845, of which 151, or 17-9%, are taken to be in the first pregnancy. 



Although the frequency curve of '0-1 corpora albicantia' conceptions is reasonably symmetrical, 

 there is a conspicuous subsidiary peak of conceptions in May and it is instructive to plot the ' o corpus 

 albicans' and ' 1 corpus albicans' groups separately for comparison. In Text-fig. 37 (top), the con- 

 ception frequencies of these two groups are plotted and also the conception curve for the ' 2 corpora 

 albicantia' group. The distribution of the 'o corpus albicans' conception frequencies is very 

 symmetrical with a sharp peak in July (mean and median dates, 23 July ; mode 26 July). When the 

 conception curve for the ' 1 corpus albicans ' group is examined, the mean and median dates are found 

 to be at the middle of July (18 July and 12 July, respectively), and the curve is skewed with the mode 

 in May, two months earlier than the peak number of ' o corpus albicans ' conceptions. This mode is 

 clearly responsible for the subsidiary peak in the ' 0-1 corpora albicantia ' conception curve. It will 

 be seen that this advancement of the peak conception date is also characteristic of the ' 2 corpora 

 albicantia ' group, in which it is slightly more marked. When similar curves are plotted for later corpora 

 albicantia groups the proportion of conceptions in April/May is seen to show a marked increase in the 

 '3 corpora albicantia' group and in later groups this becomes the peak month. Wheeler (1930) from 

 his examination of the frequency of numbers of corpora concluded that in the first sexual season the 

 majority of females become pregnant at the first ovulation, while in subsequent seasons unsuccessful 

 ovulations usually precede pregnancy. In explanation he suggested that whales nearing puberty tend 

 to stay near the breeding areas, or at any rate do not make a long southward migration, so that they 



