THE SECRETION AND PROPERTIES OF MILK 



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protoplasm numerous fat globules make their appearance. If sections be 

 made of the gland during the various stages of its activity and stained 

 by Altmann's method (acid fuchsin and picric acid), it will be seen that the 

 commencement of activity is marked by the growth of the innermost part of 

 the cells and the development in these of a number of granules (Fig. 579). 

 These granules finally lengthen into shapes like spirilla, while others of them 

 form fat and become 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 verv little about the mechanism of milk secretion. It seems 



FIG. 579. 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, etc.) 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. 



impossible at present to explain the very close adaptation between the 

 activity of the secretory cells and the needs of the infant or young 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 of the gland cells, and that the 

 lactose is derived in the same way from some sort of gluco-protein or gluco- 

 nucleo protein ; but the evidence for either of these assumptions is very scanty. 

 The growth of the mammary glands during pregnancy is largely determined 

 by some form of chemical stimulation, the specific hormone being produced in 

 the corpus luteum of the ovary and possibly also 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 con- 

 tinuous growth of the gland cells. With the expulsion of the foetus at birth 

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