578 Comparative Morphology of Chordates 



the entire gland, however deeply it may penetrate, is of epidermal 

 origin. A mouse usually has only 1 gland at each nipple; carnivores 

 have 5 or 6 per nipple; man, 15 to 20. The interstices of the glandular 

 tissue are filled by richly vascular connective tissue. 



The early embryo of a mammal develops a pair of "milk-lines," 

 each being an ectodermal thickening or ridge which extends ventro- 

 laterally from the axillary region to the groin (Fig. 447). The paired 

 mammary organs develop at intervals along these milk-lines. The 

 number and location of the definitive organs is highly variable among 

 the several orders of mammals. In general, the number varies with the 

 number of young in a litter, ranging from a maximum of 11 pairs in a 

 Madagascar hedgehog, Centetes (an insectivore) to a single pair in 

 such large mammals as the horse, elephant, whale, and man. In apes, 

 man, bats, elephant, and sea cows, the one pair is anterior or pectoral 

 in position. The one pair in the whale and horse is posterior or inguinal. 

 The cow's udder consists of the contiguous glandular masses of two 

 pairs of mammary organs. The marsupial opossum has six pairs of 

 nipples plus an odd one in the middle of the mammary region. 



The mammary organs of monotremes lack nipples. In the 

 duckbill, the several milk-ducts of each of the one pair of mammary 

 organs merely open on the flat surface of a hairless area of skin. This 

 pair of "milk-fields" is situated ventrally about midway of the length 

 of the trunk. The young licks off the milk as it exudes from the pores 

 of the ducts. In Echidna (Fig. 448 A) each of the pair of milk-fields is 

 depressed to form a shallow "mammary pocket" into which the 

 snout of the feeding young is inserted. 



All mammals other than monotremes possess nipples. A 

 mammary organ develops at a locally thickened spot on the milk-line 

 (Fig. 447A, B). Each gland is formed by a cord of ectodermal cells 

 which grows inward from the thickened region (Fig. 447C). This 

 region becomes temporarily indented to form a shallow pouch resem- 

 bling the "mammary pocket" of Echidna (Fig. 447C). In some mam- 

 mals the embryonic pouch is finally everted to become the protruding 

 nipple at whose apex open the milk -ducts (Fig. 448J3). In others the 

 integumentary rim of the pouch grows outward so that the broad and 

 shallow cavity of the pouch is transformed into a narrow and long 

 canal at whose bottom open the milk-ducts (Fig. 4481?, F). The solid 

 nipple is commonly called "true," the hollow one "false." "True" 

 nipples are found in marsupials, rodents, elephants, sea cows, and 

 primates: "false" in cetaceans, cattle and other cud-chewing ungu- 

 lates, and horses. 



Mammary glands normally become functional only in the female. 

 Their differentiation is not completed until puberty and they are 



