588 INTEGUMENTARY SYSTEM 



-SWEAT PORE 

 — EPIDE RMIS 



- '-■ "'^ ) v Si!// -' '^'' ' -•■ J>' nil VyV^ ■■ ~/ BEGINNING 



-=: .S> ^ * v^ •.- ^^ ; l\ \M -K/i-'' rf; .' - F P I E R M A L 



>^\'SJW0'P*">'"' '-■' (Sa^^'-^-vN'' INVAGINATION 



_2_r= FAT CELLS 

 1^ 



Fig. 276. Diagram of sudoriferous (sweat) gland. 



pectoral region, but it is extensive in the pig, dog, and cat. In the cow, horse, 

 deer, etc., its greatest development is in the inguinal area. 



Only very restricted areas of each mammary ridge on either side are utilized 

 in mammary-gland development. In the pig or dog embryo, a series of local- 

 ized thickenings begin to appear along the ridge. In the sheep, cow, and 

 horse, these thickenings are confined to the inguinal region, whereas in the 

 primates and the elephant, they are found in the pectoral area. In the human, 

 one thickening in each ridge generally appears, although occasionally several 

 may arise. These thickenings represent the beginnings of the nipples and 

 result from increased proliferations of cells (fig. 277B). Eventually, each 

 thickened portion of the ridge becomes bulbous and sinks inward into the 

 dermis (fig. 277C). Gradually, solid cords of cells push out from the lower 

 rim of the solid epidermal mass into the surrounding dermal tissue (fig. 277D). 

 These cords of cells represent the rudiments of the mammary-gland ducts. 

 Secondary outpushings appear at the distal ends of the primary ducts. Later, 

 lumina appear in the primary ducts. Further development of these ducts, with 

 the formation of the terminal rudimentary acini, occurs during late fetal stages, 

 resulting in the formation of an infantile state. This condition is found at 

 birth in the human, dog, cat, etc. Under the influence of hormones present 

 in the blood stream of the mother (see Chap. 2, p. 1 03 ) , these acini may secrete 

 the so-called "witch's" milk in the newborn human male and female. While 

 the occurrence of this type of milk secretion is not uncommon, the gland as 

 a whole is in a rudimentary, undeveloped state. It remains in this infantile 

 condition until the period of sexual development when, in the female, the 

 mammary-gland ducts and attendant structures begin to grow and develop 

 under the influence of estrogen, the female sex hormone. (See Chap. 2.) It 

 should be observed that the rounded condition of the developing breast in 

 the human female at the time of puberty (fig. 277F) is due largely to the 

 accumulation of fat and connective tissue and not to a great extension of the 

 duct system of the glands, although some duct extension does occur at this time. 



As the original epithelial thickening of the nipple rudiment sinks inward, 



