. The chordates. Chordata. 580 Comparative Morphology of Chordates functional only during the period of rearing young. In males, they are present in a rudimentary state. The wide range of difference in number and position of the mam- mary organs in the several groups of mammals is, in some degree, made intelligible by the common occurrence of the milk-lines in mam- malian embryos. The milk-line apparently is potentially mammary throughout its entire length. In a given species it produces organs whose number and position are appropriate in relation to the number. Fig. 449. The occasional presen
. The chordates. Chordata. 580 Comparative Morphology of Chordates functional only during the period of rearing young. In males, they are present in a rudimentary state. The wide range of difference in number and position of the mam- mary organs in the several groups of mammals is, in some degree, made intelligible by the common occurrence of the milk-lines in mam- malian embryos. The milk-line apparently is potentially mammary throughout its entire length. In a given species it produces organs whose number and position are appropriate in relation to the number. Fig. 449. The occasional presence of supernumerary teats (polymastism) in man indicates the genetic continuity of man and other mammals. (After Wiedersheim. Cour- tesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) of young and the conditions under which suckling takes place in that species. Within a species, especially one which normally has a large number of mammary organs, there is commonly some variation in the number. Sometimes, even in man, and from unknown causes, the milk- lines produce organs in excess of the normal number and in unusual localities. In the human female the "supernumerary" organs, situated usually above the normal pair, may become functional (polymas- tism). In human males supernumerary nipples are not uncommon (polythelism), even to such extreme overproduction as that of the case represented in Fig. 449. In its highly glandular character mammalian skin resembles am- phibian rather than reptilian skin. It is possible that the very early. 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 Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : Blakiston
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