THE INTEGUMENTARY SYSTEM 



127 



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 



CUT LIMB BUD 

 I POSITION OF DEFINITIVE GLAND 



'I?^|f1"^^^ MILK LINE 



GLAND ANLAGE 



NIPPLE 



EPIDERMIS V u-^' 

 SMOOTH MUSCLE.ltj_ 



CORIUM- ■ 



MUSCLE 



13 5 MM. HUMAN EMBRYO B MUSCLE C FAT 



Fig. 118. — 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 a.xillary 

 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 tubtilar gland. (Redrawn from Arey, after Tourneux.) 



Fig. 119. — Scheme of different kinds of nipples. Single line, ordinary integument; 

 double line, that of primary mammary pocket. A, primitive condition, found in 

 Echidna; B, human nipple; D, Didelphys before lactation; C, same at lactation; E, 

 embryonic, F. adult conditions in cow. B and C are true nipples, F, a pseudo-nipple 

 (teat). (Based on figures by Weber from Kingsley.) 



depression, the "mammary pocket", surrounded by a fold of skin. From 

 this condition in monotremes, the teats of the higher mammals have 

 evolved, either by elevating the milk-field at the bottom of the pocket 



