MAMMARY GLANDS 



297 



MAMMARY GLANDS 



The tubular mammary glands are peculiar to mammals. In embryos 

 of 9 mm. (Figs. 94 and 118) an ectodermal thickening extends ventro-later- 

 ally between the bases of the limb buds on either side. This linear 

 epidermal thickening is the milk line. In the future pectoral region of this 

 line, by the thickening and downgrowth of the epidermis, there is formed 

 the papilla-like anlage of the mammary gland (Fig. 302 A). From this 

 epithelial anlage buds appear (B) which elongate and form solid cords, 

 15 to 20 in number,' the anlages of the milk ducts (C). These branch in 

 the mesenchymal tissue of the corium and eventually produce the alveolar 



Gland anlage 



Epidermis 



Epidermis Gland anlage 



Nipple 



Pectoral 

 muscle 



Duel 



Smooth muscle 

 of areola 



Panniculus 

 adiposus 



FIG. 302. Sections representing three stages in the development of the human mammary 

 gland (Tourney). A, fetus of 32 mm.; B, of 102 mm.; C, of 244 mm. *, Groovejlimiting 

 glandular area; 



end pieces of the mammary glands. In the region where the milk ducts 

 open' on the surface the epidermis is evaginated to form the nipple. The 

 glands yield a little secretion ('witch milk') at birth; they enlarge rap- 

 idly at puberty and are further augmented during pregnancy, while two 

 or three days after parturition they become functionally active. 



The mammary glands are regarded as modified sweat glands. This homology is made 

 because their development is similar, and because in the lower mammals their structure is 

 the same. Rudimentary mammary glands (of Montgomery), which also resemble sweat 

 glands, occur in the areola about the nipple. In many mammals, numerous pairs of mam- 

 mary glands are developed along the milk line (pig, dog, etc.) ; in some a pair of glands is 

 developed in the pectoral region (primates, elephants) ; in others, glands are confined to the 

 inguinal region (sheep, cow, horse). 



Anomalies. Supernumerary mammary glands (hypermastia) or nipples (hy perihelia) 

 are not infrequent between the axilla and groin. These represent independent differen- 

 tiations along the primitive milk line. 



