. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. E F • Fig. 164. Development of the mammary glands. A, diagrammatic cross section through the milk-line (Fig. 163a); b, the epidermal milk-line, after breaking up into a chain of isolated milk-hills (Fig. 163b) has sunk down into the corium (dotted) and is no longer apparent externally, thus forming the milk-field stage; the levelled area, where the milk-hills were, becomes depressed, c, forming the milk-pocket stage. The sunken epidermal plug penetrates st
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. E F • Fig. 164. Development of the mammary glands. A, diagrammatic cross section through the milk-line (Fig. 163a); b, the epidermal milk-line, after breaking up into a chain of isolated milk-hills (Fig. 163b) has sunk down into the corium (dotted) and is no longer apparent externally, thus forming the milk-field stage; the levelled area, where the milk-hills were, becomes depressed, c, forming the milk-pocket stage. The sunken epidermal plug penetrates still deeper into the corium, giving rise to the mammary glands, d, preliminary indifferent milk-pocket stage, with the two longer arrows indicating the direction of epidermal growth that results in the formation of a "false nipple," while the short arrow shows how the "true nipple" forms; e, false nipple, with ducts of the mammary gland opening at the bottom of a milk-canal; f, true nipple, with the mammary ducts opening directly at the tip. In man as well as other mammals, extra nipples (hyperthelism) (Fig. 165) not infrequently occur, as also do extra breasts (hypermastism) (Fig. 166). Such persistent embryonic relics, particularly in the case of hyper- thelism, occur quite as often in males as in females. Usually these super- numerary parts are arranged along the vanished embryonic milk-line. The mammary apparatus of monotremes presents many exceptions to that of other mammals. Instead of being alveolar in form the mammary. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Walter, Herbert Eugene, b. 1867; Sayles, Leonard Perkins, 1902-. New York : Macmillan Co.
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