4 io DISCOVERY REPORTS 



Lactating females with one corpus albicans in the ovaries will be in their first lactation and in the 

 material there are seven such records. The criterion of full lactation which is adopted here is discussed 

 below (p. 444). During the first lactation the gland undergoes an apparently irreversible change; 

 it may enlarge to over 20 cm. in thickness (mean 16-1 cm., range 8-24 cm.) and although it involutes 

 after weaning (or death) of the calf it does not revert to the former immature condition. 



In females which have previously experienced at least one lactation period the mammary glands are 

 either immediately post-lactation in condition or what Mackintosh and Wheeler (1929) termed 

 'resting' or 'intermediate' between this condition and lactation (see below, p. 444). The mammary 

 glands are then usually more than 3 cm. deep (from 2-5 to 14 cm., Text-fig. 35) and there are brown 

 lobes of coiled ducts set in a thick connective tissue framework (Mackintosh and Wheeler, 1929, 



25 



- 15 



X 



10 



10 20 o 10 so o 10 



PUBERTY PREGNANCY LACTATION 



PRIMIPAROUS 



SECOND FULL 



PREGNANCY LACTATION 



END OF 



LACTATION 



MULTIPAROUS - 



Text-fig. 35. Frequency distribution of mammary gland depth in several groups of female fin whales. 



fig. 138; van Lennep and van Utrecht, 1953, fig. 1). Apart from the usually greater thickness, the colour 

 and appearance of ' resting ' or ' intermediate ' mammary glands is quite different from that of the 

 gland which has not yet been functional. The gross appearance of the mammary gland in section is, 

 therefore, considered to be a reliable criterion of nulliparous or primiparous females. 



The first pregnancy 



There are 88 pregnant fin whale females in the material which can be classed as primiparous on the 

 grounds outlined above. This figure includes only those individuals for which data on both mammary 

 development and ovaries are available; it excludes females known to be primiparous because they 

 are pregnant with only one corpus luteum and no corpus albicans in the ovaries, but for which no 

 data on the mammary gland condition are available. Of these females, 60 (68%) had one corpus 

 luteum and no corpus albicans, 20 (23 %) had one corpus luteum and one corpus albicans, 7 (8 %) had 

 one corpus luteum and two corpora albicantia, and one (1 %) had one corpus luteum and three corpora 

 albicantia. They had, therefore, become pregnant for the first time at the first, second, third, or fourth 



