512 Dr. J. E. Lane-Claypon and Prof! E. H. Starling. [Feb. 12, 



about 2 to 3 cm. in diameter, surrounding the situation of each nipple (fig. 2). 

 On section the gland is seen to consist still merely of ducts. These however 

 are in an active state of proliferation, throwing out tree-like branches towards 

 the periphery. The epithelial lining of the ducts is two or three cells 

 thick ; the cells are much more swollen than in the virgin gland, and 

 numerous mitotic figures are seen in the epithelium. This growth proceeds 

 rapidly, so that about the ninth day, on reflecting the skin from the abdomen, 

 the whole is found to be covered with a thin layer of gland tissue. The 

 margins of the glands are now almost contiguous, and the glands vary in 

 diameter from 5 to 8 cm. (fig. 3). The appearance of the branching ducts in 

 the stained specimen is also somewhat different, the ducts presenting, 

 especially towards the periphery of the gland, cauliflower-like excrescences 

 from their margins. The extreme periphery of the gland is generally some- 

 what thicker than the rest of the gland. On section it is found, especially 

 at the periphery, that the formation of alveoli is commencing. This formation 

 of alveoli proceeds henceforth rapidly, together with a continued growth of 

 the ducts. At the twenty-fifth day the whole surface of the abdomen is 

 covered with a layer of gland tissue, which may be 0'5 cm. thick, and on 

 section is found to be formed for the greater part of alveoli. About this date, 

 too, fat globules are forming in the cells of the alveoli. In the rabbit 

 pregnancy lasts about 30 days. During the greater part of pregnancy, from 

 the ninth day to the twenty-fifth, a watery fluid can be squeezed from the 

 nipples. During the last two or three days of pregnancy this fluid becomes 

 milky in character, so that immediately after parturition the mammary 

 glands are already full of milk. 



THE ARTIFICIAL PRODUCTION OF GROWTH IN THE MAMMARY GLANDS. 

 We may now proceed to an account of the experiments in which we tried 

 to reproduce artificially some of the remarkable changes which are normally 

 determined by pregnancy. It was evident to us, before we began our 

 researches, that it would be difficult, if not impossible, to present any 

 stimulus to the mammary glands which would be as effective as the normal 

 one. For, wherever the mammary hormone is manufactured, the manu- 

 facture must be assumed to proceed continuously. There is therefore a. 

 constant leakage of the active substance into the blood, and it is probable 

 that the amount of this substance produced increases with the duration of 

 pregnancy. At no time will the mammary gland be set free from the 

 influence of this specific stimulus. On the other hand, however, we might 

 prepare our extracts of the tissues, we could not expect to get more than the 

 amount residual in the tissue and caught, so to speak, in its progress through 



