THE REPRODUCTIVE ORGANS 



403 



of the milk. Sometimes the outlines of these droplets is in the form of a complete 

 circle ; at other times (possibly when the gland has been emptied by suckling) the outlines 

 are broken up or fragmentary. Where an osmic acid fixative is used the droplets are 

 densely blackened, no doubt owing to the very high percentage of fat which is present 

 in whale's milk. In such cases the droplets within the lumen of the cells are similarly 

 blackened. 



The depth of the mammary gland during the secretion of milk varies in Blue and 

 Fin whales between about 15 and 30 cm. 



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Fig. 135. Section of the immature 

 mammary gland. 



Fig. 136. Section of mammary gland 

 in functional activity. 



The intermediate stage occurs in certain sexually mature whales in which the 

 mammary glands are not functionally enlarged. It has been seen that during lactation 

 the lobules become greatly swollen and compress the intervening connective tissue 

 into the smallest possible space. In the intermediate stage the lobules appear either 

 to have started swelling in preparation for lactation, or, more often, are in the process 

 of contracting after the end of lactation. This condition is never found in pregnant 

 whales except at the very end of gestation, but it is curious that it occurs more often 

 in "resting" whales than the normal resting condition which is almost always present 

 in pregnant whales. The lobules are still large (Fig. 137), and, though noticeably 

 smaller than in lactating whales, they are considerably better developed than in the 

 resting stage. The connective tissue space is still restricted, but the alveoli are shrivelled 

 and smaller than in the lactating gland, their outline is less easily traced, and their 

 lumen, at least in haematoxylin-eosin preparations, is not easy to distinguish. Droplets 

 are practically absent from the alveoli, though one or two may sometimes be visible 



19-2 



