444 DISCOVERY REPORTS 



in Balaenopterids not only oestrus and ovulation, but occasionally mating and conception, may occur 

 in polar waters in the summer months. It would appear that at most only about 6% of all conceptions 

 occur in this way (Table n, October-March conceptions). This is correlated with the observation 

 that the great majority of male fin whales examined in antarctic waters are found to be in full anoestrus 



(P- 39 6 )- 



The lactation period 



Text-fig. 35 (p. 410) illustrates the cycle of activity of the mammary gland. Attention has already been 

 drawn to the relative thinness of the mammary glands of nulliparous and primiparous females, and to 

 the fact that after its expansion during the first lactation period the mammary gland does not return to its 

 former size, but decreases to a depth of about 5 cm. in primiparous ' resting ' females and in second- 

 pregnancy females. This figure also suggests that in lactating multiparous females the mammary 

 gland is rather thicker than in primiparous females, although the data are too small to allow definite 

 conclusions to be drawn. 



In multiparous females the mammary gland in full lactation averages 20 cm. in depth (range 

 10-30 cm.); the lobules of glandular tissue with expanded alveoli are large, and the connective tissue 

 is arranged as relatively thin supporting septa. In pregnant females and ' resting ' females which have 

 experienced at least one lactation period, the glands are either involuted or intermediate between this 

 condition and the lactating condition. The 'intermediate' glands are then about 7-1 1 cm. thick, 

 brownish in colour with large lobes of alveolar tissue and relatively small amounts of connective tissue 

 (Mackintosh and Wheeler, 1929, fig. 137). In the involuted mammary gland the brownish alveolar 

 lobes are smaller than in the ' intermediate ' gland, and the connective tissue framework is corre- 

 spondingly more conspicuous (Mackintosh and Wheeler, 1929, fig. 138); the thickness of the gland is 

 usually from 4 to 8 cm. There is little chance of confusing a gland in this condition with an immature 

 gland, because both thickness and colour are usually very different. 



Another group is shown in Text-fig. 35, which has been termed the 'end of lactation' group, and 

 some explanation is necessary. Chittleborough (1958) noted that after lactation has ended, when 

 involution of the gland is well-marked, there may be liquid in the lacteal ducts, which is usually a 

 whitish or turbid yellowish thin fluid. There are 22 fin whales in the present material which have 

 mammary glands in this condition. They are considered to be involuting after the successful termina- 

 tion of lactation or following loss of the calf. They range in thickness from 7 to 17 cm. (mean 1 1 -i cm.) 

 and are shown in Text-fig. 35 as an 'end of lactation' group. 



It will be noted that the upper part of the histograms showing mammary gland depth are closely 

 similar in both the ' resting ' and pregnant groups of females, and overlap the values for the ' end of 

 lactation' group, and even the two lowest values in the lactating group. This is to be expected in the 

 case of the resting group which normally follow on after weaning; the thicker glands in the pregnant 

 group must similarly be considered to represent females which became pregnant at a post-partum 

 or post-lactation ovulation. The estimated foetal ages for these females agree with this interpretation. 



The criterion of lactation adopted here is the presence, in the mammary glands or ducts, of milk 

 which is apparently normal in colour and consistency. Usually the appearance of the cut gland is 

 also diagnostic. The quantity of milk present is not important, because it will vary according to the time 

 elapsed since the calf was last fed. As was noted above the presence of milk is usually, but not invariably, 

 associated with mammary glands which are thicker than those of non-lactating females (Text-fig. 35). 



However, the presence of milk or gross appearance of the gland are not completely valid criteria 

 of lactation because some females diagnosed as being in full lactation are found, on histological 

 examination, to have stopped active secretion of milk. Van Lennep and van Utrecht (1953) have shown 

 that of 69 females (mainly blue and fin whales) said on these grounds to be lactating, histological 



