1392 PHYSIOLOGY 



metamorphosed into fat granules. The nuclei of the cells also divide, 

 apparently in preparation for the replacement of some cells which 

 undergo complete degeneration and are cast off into the secretion. 



We know very little about the mechanism of milk secretion. It 

 seems impossible at present to explain the very close adaptation 

 between the activity of the secretory cells and the needs of the infant 

 orlyoung animal. Two at least of the constituents of milk, caseinogen 

 and lactose, are peculiar to this secretion. It has been assumed that the 

 caseinogen is produced by some sort of alteration in the nucleo-proteins 



V 



FIG. 564. Sections of mammary gland of guinea-pig (fat-granules 



stained black with osmic acid). 



A, during rest. B, during active secretion. It will be noticed that in this 

 case the active formation of products of cell-metabolism (granules, &c.) begins 

 with the commencement of secretion, and does not occur almost exclusively 

 during rest, as in the salivary glands. In the mammary gland, the active 

 growth of protoplasm, the formation of granules from the protoplasm, and 

 the discharge of these granules in the secretion appear to go on at one and 

 the same time. 



of the gland-cells, and that the lactose is derived in the same way from 

 some sort of gluco-protein or gluco-nucleoprotein, but the evidence for 

 either of these assumptions is very scanty. The growth of the mam- 

 mary glands during pregnancy is largely determined by some form of 

 chemical stimulation, the specific hormone being produced first in the 

 ovary and secondly in the growing foetus.* It has been suggested by 

 Hildebrandt that this stimulus is inhibitory in character inhibitory, 

 that is to say, of secretion and therefore tending to the continuous 

 growth of the gland-cells. With the removal of the foetus at birth the 

 source of the inhibitory stimulus is removed and the overgrown gland- 



* But see p. 1363. 



