THE FEMALE REPRODUCTIVE ORGANS. 891 



bodies of many fetuses, when injected repeatedly into a virgin rabbit 

 caused a genuine development of the mammary glands closely 

 simulating the growth that normally occurs during pregnancy. 

 Since similar extracts made from ovaries, placental and uterine 

 tissues had no effect, they conclude that a specific chemical sub- 

 stance (a hormone) is produced in the fetus itself and, after absorp- 

 tion into the maternal blood, it acts upon the mammary gland, stim- 

 ulating it to growth. Since the birth of the fetus is followed by 

 active secretion in the mammary glands they adopt further the 

 view that this substance, while promoting the growth of the gland 

 tissue, inhibits the catabolic processes which lead to the formation 

 of the secretion. With the birth of the fetus this substance is 

 withdrawn and secretion begins, and, on the contrary, the secretion 

 is suspended when a new pregnancy is well advanced. 



As was said in speaking of the histology of the gland, the se- 

 creting alveoli are not fully formed until the first pregnancy. Dur- 

 ing the period of gestation the epithelial cells multiply, the alveoli 

 are formed, and after parturition secretion begins. As the liquid 

 is formed it accumulates in the enlarged galactophorous ducts, and 

 after the tension has reached a certain point further secretion 

 is apparently inhibited. If the ducts are emptied, by the infant 

 or otherwise, a new secretion begins. The emptying of the ducts, 

 in fact, seems to constitute the normal physiological stimulus to 

 the gland-cells, but how this act affects the secreting cells, whether 

 reflexly or directly, is not known. 



Composition of the Milk. The composition of milk is com- 

 plex and variable. The important constituents are the fats, held 

 in emulsion as minute oil droplets and consisting chiefly of olein 

 and palmitin; casein, a nucleo-albumin which clots under the in- 

 fluence of rennin; milk-albumin or lactalbumin, a proteid resem- 

 bling serum-albumin; lactoglobulin; lactose or milk-sugar; lecithin, 

 cholesterin, phosphocarnic acid, urea, creatin, citric acid, enzymes, 

 and mineral salts. It is well known also that many foreign sub- 

 stances drugs, flavors, etc. introduced with the food are secreted 

 in the milk. An average composition is: proteids, 1 to 2 per cent.; 

 fats, 3 to 4 per cent.; sugar, 6 to 7 per cent.; salts, 0.1 to 0.2 per 

 cent. The fact that casein and milk-sugar do not exist preformed 

 in the blood is an argument in favor of the view that they are formed 

 by the secretory metabolism of the gland cells. The special com- 

 position of the milk-fat and the histological appearance of the 

 gland cells during secretion lead to the view that the fat is also 

 constructed within the gland itself. Bunge has called attention 

 to the fact that the inorganic salts of milk differ quantitatively 

 from those in the blood-plasma and resemble closely the propor- 

 tions found in the body of the young animal, thus indicating an 



