MULTIPAROUS FEMALES 431 



lactation stages of the sexual cycle which have ovulated once only (Table 15). Thus in the first lactation 

 period a substantial proportion of females do not experience a post-partum ovulation, and in these 

 females there is only one corpus albicans, derived from the former corpus luteum of pregnancy. This 

 suggests that the first of the two postulates in the previous paragraph, namely that only a small 

 proportion of females experience a post-partum heat, is correct. It is, however, possible that the 

 sexual cycle of primiparous females differs in this respect from that of multiparous females (i.e. that 

 they are less fertile) and the evidence now to be presented suggests that this is so and that the majority 

 of multiparous females experience a post-partum heat. 



This evidence concerns first, the sizes of corpora in the ovaries of lactating, ' resting ' and pregnant 

 females, and secondly, the morphology and histology of certain corpora in the ovaries of lactating females. 



Measurements of corpora diameters made at South Georgia between 1925 and 193 1 comprise the 

 only consistent series which contains a fairly large number of lactating females. The methods of 

 measurement adopted by later workers vary, and since the methods were standardized in 1954 only 

 a relatively small number (32) of lactating females have been examined. Although the pelagic data 

 nearly double the size of the sample they also increase the variability, and only the South Georgia 

 material is used in the following discussion. Females which are simultaneously lactating and pregnant 

 are not included, for obvious reasons, and the remainder of the females for which corpora measure- 

 ments are available have been split into three groups according to whether they are pregnant, lactating, 

 or ' resting '. The frequency distributions of the diameters of the largest, second largest, third largest, 

 ...up to the sixth largest corpora albicantia, have been obtained. From these the mean diameters 

 and standard errors have been calculated and are shown in Table 20. 



Table 20. Mean diameter ±2 s.E. of corpus luteum and six largest corpora albicantia in the ovaries of 

 201 pregnant, 48 lactating and 59 'resting' females. The size of the corpus luteum is based upon a 

 much larger sample (p. 356) 



Corpora albicantia 



Sexual Corpus r — * > 



condition luteum 123456 



Pregnant 11-4410-15 5-04 + 0-13 4-0510-11 3-5910-11 3-4310-13 3-13 +0-19 3-0110-20 



Lactating — 5-5810-20 4-3310-22 4-1510-18 3-3610-20 3-2810-31 3-0010-53 



'Resting' — 5-5710-26 4-4210-22 3-98 + 0-24 3-6210-20 3-2010-20 3-1010-31 



In Text-fig. 45 the results are presented graphically and each group has been plotted twice so as 

 to cover two reproductive cycles, or four years. It should be noted that this represents the average 

 reproductive pattern. Some of the pregnant group will, however, be animals which conceived as a 

 result of a post-partum ovulation, and therefore did not pass through the lactating and resting stages 

 before becoming pregnant as in the majority of cases. Furthermore, the group of resting females 

 includes some which have recently ovulated (see below, p. 436) and have a young corpus albicans in 

 the early rapid phase of regression, while in others this most recently formed corpus albicans has 

 undergone a much longer period of regression. These variations are a complication in the interpreta- 

 tion of the size distribution of corpora albicantia in this group. 



One assumption is made, that with increasing age the corpus albicans progressively shrinks in size. 

 Consequently, if say the third largest corpus albicans in one group is larger than the third largest 

 corpus in the preceding group then the former cannot have been derived from the latter. Similarly, 

 if these two corpora are equal in size then one is unlikely to have been derived from the other because 

 it should have regressed by an appreciable amount in the intervening months. It is necessary again 

 to emphasize that the corpus luteum of pregnancy in the fin whale is much larger than the corpus 

 luteum of ovulation (11-44 cm - as compared with 8-28 cm.). Therefore, among corpora albicantia of 



