402 



HISTOLOGY 



the ectodermal proliferation. There are ultimately from 15 to 20 of these 

 tn each breast, and they branch as they extend through the connective tis- 

 sue. At birth the nipple has become everted, making an elevation, and 

 at that time the glands in either sex may discharge a little milky secretion 

 similar to the colostrum which precedes lactation. The glands grow in both 

 sexes until puberty, when those in the male atrophy and only the main 

 ducts persist. In the female enlarged terminal alveoli are scarcely evi- 

 dent until pregnancy. The glands until then are discoid masses of connec- 

 tive tissue and fat cells, showing in sections small scattered groups of duct- 

 like tubes. 



Toward the end of pregnancy each of the fifteen or twenty branched 

 glands forms a mammary lobe, and its alveolo-tubular end pieces are 



Branch of an excretory duct. Connective tissue. 





' '~~; - - ' - -' --_-/-" 



FIG. 414. SECTION OF A HUMAN MAMMARY GLAND AT THE PERIOD OF LACTATION. X 50. 



grouped in lobules. The secretory epithelium is a simple cuboidal or 

 flattened layer, in which fat accumulates at the seventh or eight month of 

 pregnancy. The fat first appears as small granules at the basal ends of 

 the cells, where it is taken up from the surrounding tissue. It is not pro- 

 duced by the gland cells. Leucocytes, derived from the connective tissue, 

 make their way between the epithelial cells of the alveoli and enter the 

 gland lumen, where some of them degenerate; others receive fat from the 

 gland cells, either in solution, or in drops which are devoured by phagocytic 

 action. These fatty leucocytes grow to considerable size and are called 

 colostrum corpuscles. Beneath the alveolar epithelium there are basal or 

 basket cells, which have been compared with the muscle fibers of sweat 



