298 



HISTOGENESIS 



extends ventrolaterally between the bases of the limb buds on either side. This 

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

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

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

 anlage buds appear {B) which elongate and form soHd 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 end pieces of the mammary glands. 

 In the region where the milk ducts open on the surface the epidermis is evagi- 



Gland anlage 



Epidermis 



Nipple 



Epidermis Gland anlage 



'i-ifh'^ u\£ •'-■'«»5 muscle 

 B 



isL J_. Smooth muscle 

 S!a I of areola 



i 



Pannicuhis 

 adiposus 



Fig. 302. — Sections representing three successive stages of development of the human mammary 

 gland (Tourneux). A, fetus of 32 mm.; B, of 102 mm.; C, of 244 mm. *, Groove limiting glandular 

 area. 



nated to form the nipple. The glands enlarge rapidly at puberty and are further 

 augmented during pregnancy, while after parturition they become functionally 

 active. 



The mammary glands are homologised with sweat glands because their development is 

 similar, and because in the lower mammals their structure is the same. Rudimentary mam- 

 mary glands (of Montgomery), which also resemble sweat glands, occur in the areola about 

 the nipple. In many mammals numerous pairs of mammary glands are developed along the 

 milk Hne (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). In man 

 supernumerary mammary glands developed along the milk line are of not infrequent occur- 

 rence. 



THE NAILS 



The anlages of the nails proper are derived from the epidermis and may be 

 recognized in fetuses of 45 mm. (C R) . A nail anlage forms on the dorsum of each 

 digit and extends from the tip of the digit almost to the articulation of the terminal 

 phalanx. At the base of the anlage, that is, proximally, the epidermis is folded 

 inward to form the proximal nail fold (posterior nail fold of the adult) (Fig. 303 



