402 DISCOVERY REPORTS 



the gland is in full activity the swelling is usually distinguishable externally (Plate XXX, 

 fig. i), and is very evident after the blubber has been removed from it (Plate XXXV, 

 fig. 3). When milk is not being secreted the outline may be almost indistinguishable 

 even after flensing. 



Several large ducts run longitudinally through the gland. These are fed by numerous 

 smaller ducts and become enlarged posteriorly to form sinuses or reservoirs for the 

 milk. They join finally in one large sinus which communicates with the teat. 



If the gland of a sexually mature whale which is not lactating is cut across, it is seen 

 to consist of numerous lobes subdivided into small pinkish lobules, with ducts of all 

 sizes and blood-vessels. There may be some variation in the amount of blood which 

 is present. These pink lobules are not seen in immature whales, but the connective 

 tissue in which they subsequently develop can be distinguished quite easily. 



When examined histologically the gland in whales does not appear to differ in any 

 essential from that of other mammals. The greater size of the gland appears to be 

 allowed for by increased numbers of the alveoli in a lobule, and multiple subdivision 

 of the lobes of the gland, rather than by any diff'erent structure. 



It will be convenient to describe first the immature gland. This shows the same 

 structure both in the foetus and in large though still sexually immature whales. When 

 examined histologically it is found to consist mainly of connective tissue in which a 

 few ducts and blood-vessels are seen, of which the former are surrounded by clusters 

 of cells forming imperfect alveoli grouped together in small lobules (Fig. 135). In 

 some cases (such as adult whales which have not yet been pregnant) the distinction 

 between the immature and mature condition is not very sharp, but as a rule there is 

 no difficulty in recognizing the immature type. 



Among sexually mature whales the gland may be found in no less than four difi^erent 

 conditions. These are as follows: 



1. Lactating, in which milk is being actively secreted. 



2. Intermediate , in which the lobules of the gland are better developed than in the 

 resting condition, but less than in the lactating condition. This condition appears to 

 occur immediately before lactation and again in the apparently prolonged involution 

 of the gland afterwards. 



3. Resting, in which complete involution appears to have taken place. 



4. Virgin, which occurs in a few young adults which have probably never been 

 pregnant. 



In the lactating gland (Fig. 136) the lobules are greatly swollen, and the space 

 between them which is occupied by connective tissue is considerably restricted. The 

 alveoli are distended and their outline is rounded and relatively distinct. Droplets of 

 secretion are clearly seen in the lumen of the alveolar cells which are noticeably swollen 

 and have small, densely staining nuclei. The lumen of the alveoli is filled with larger 

 droplets, the size of which, however, may vary. This variation might possibly depend 

 on the freshness of the whale when the tissue was fixed, or it might be correlated with 

 the rate at which the secretion is being drawn off, or with variations in the constitution 



