THE INTEGUMENTARY SYSTEM 



l8l 



membrane. The light is produced by the oxidation of luciferin secreted 

 by the gland. No carbonic acid and little heat are evolved in the process. 



Fig. 138. — The presence of supernumerary teats (polymastism) in man supports 

 the theory of the animal origin of the human body. Their repeated occurrence has 

 received no other rational explanation. They are reversions or atavisms. (Redrawn 

 after Wiedersheim.) 



CUT LIMB BUD 



POSITION OF DEFINITIVE GLAND 



GLAND ANLAGE 



NIPPLE 



EPIDERMIS ^i J 



SMOOTH MUSCLE. ttLa. ci^^i 



rCORIUM — 



laSMM. HUMAN EMBRYQ B MUSCLE C FAT 



Fig. 139. — A figure illustrating the development of the mammary gland in man. 

 A 13.5 mm. embryo shows the "milk line", a ridge which extends from the axillary 

 region to the groin. The definitive gland develops only from the anterior portion 

 of this line. Taken with the evidence of supernumerary teats in man, the line is 

 interpreted as proof that the ancestors of man had more than a single pair of mammary 

 glands. (Redrawn from Arey, after Kollmann.) 



A, B, and C are sections of the definitive mammary gland in successive stages of 

 ontogenesis. A is from a two-months embryo, B from a four-months embryo, and 

 C from a seven-months embryo. From its development the mammary gland is 

 seen to be a compound tubular gland. (Redrawn from Arey, after Tourneux.) 



Mammary Glands. Mammary glands first appear in monotremes, 

 as a pair of milk-secreting organs on the ventral side of the body. They 

 are without nipples, and in Echidna they pour their secretion into a 



