1906.] Growth and Activity of the Mammary Glands. 511 



probably occurred a considerable time, a week or more, previous to its 

 expulsion, and yet the appearance of milk in the gland has only taken place 

 two or three days after the actual labour. Halban concludes, though, as it 

 seems to us, on insufficient grounds, that the difference between the two sets 

 of cases depends on whether the death of the placenta accompanies that of 

 the foetus, and locates the seat of production of the specific hormone not in 

 the foetus, but in the chorionic villi and placenta. 



It is evident that no satisfactory solution of this question can be arrived at 

 except by experiment. If any of the organs we have mentioned is the seat 

 of production of a hormone which determines the growth of the mammary 

 gland, it should be possible to obtain this hormone by extraction of the organ, 

 and by its injection into a virgin animal produce the changes in the mammary 

 glands which are characteristic of pregnancy. It is on these lines that we 

 have proceeded in the present research, and have chosen rabbits as subjects 

 of experiment since they can be obtained in large numbers, are easily 

 handled, and breed fairly well in confinement. Before describing the 

 experiments themselves we must say a few words as to the normal course of 

 development of the mammary gland in this animal during pregnancy. 



The Natural Growth of the Mammary Glands. 



In a virgin rabbit of eight months to a year old, the average age of the 

 rabbits used in our experiments, it is difficult with the naked eye to see any 

 trace of the mammary gland in the tissue lying under the nipples. In order 

 to bring the gland into view we have removed the skin with the subcutaneous 

 tissue, dissected the latter away from the skin, stretched it on a ring of cork, 

 and then, after hardening, stained it in a weak solution of hematoxylin. 

 After staining, the subcutaneous muscle fibres were dissected away, leaving 

 only connective tissue, with auy mammary gland tissue which might be 

 present, surrounding the nipple. In the spread-out specimen it is possible to 

 see in the immediate neighbourhood of the nipple the ducts which form the 

 mammary gland. As a rule they are limited to an area not more than 1 cm. 

 broad. Plate 19, fig. 1, shows the largest extent of mammary gland we 

 have ever observed in a virgin rabbit of this age. On section the gland is 

 found to consist entirely of ducts, which are lined with a single layer of 

 flattened epithelial cells, and terminate blindly. In no case is there any trace 

 of the alveoli which are so characteristic of the fully-formed gland. 



With the occurrence of conception a marked change begins in the gland. 

 Four or five days after conception, when it is still impossible with the naked 

 eye to discover any embryos in the swollen uterine horns, on reflecting the 

 skin from the abdomen each mammary gland appears as a circular pink area, 



