328 



HISTOLOGY. 



MAMMARY GLANDS. 



In young mammalian embryos generally, the mammary glands are 

 first indicated by a thickened line of ectoderm extending from the axilla 

 to the groin. Later much of the line disappears, leaving a succession of 

 nodular thickenings corresponding with the nipples. In some mammals 

 this row of nipples remains, in others only the inguinal thickenings, 

 and in still others only those toward the axilla. Thus in man there is 

 normally only one nipple on each side. In an embryo of 25 cms. (Fig. 

 381) several solid cords have grown out from the ectodermal proliferation. 

 There are ultimately from 15 to 20 of these in each breast and they branch 

 as they extend through the connective tissue. 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 



FIG. 381. SECTION THROUGH THE MAMMARY GLAND OF AN EMBRYO OF 25 CMS. 

 I, Connective tissue of the gland. (After Basch, from McMurrich.) 



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 evident until pregnancy. The 

 glands until then are discoid masses of connective tissue and fat cells, 

 showing in sections small scattered groups of duct-like tubes. 



Toward the end of pregnancy each of the 15 or 20 branched glands 

 forms a mammary lobe and its alveolo-tubular end pieces are grouped in 

 lobules. The secretory epithelium is a simple cuboidal or flattened layer 

 in which fat accumulates at the seventh or eighth month. It first appears 

 as granules at the basal end of the cell, where it is received in combination 

 from the surrounding tissue. This fat is not produced by the gland cell. 

 The lumen of the alveoli contains leucocytes which have passed between 

 the epithelial cells, from the connective tissue. Some of them degen- 

 erate; others receive fat from the gland cells, either in combination, or in 

 drops which are devoured by phagocytic action. The fatty leucocytes 



