. The biology of the frog . ese granules swell up, and become con- 184 THE BIOLOGY OF THE FROG verted into a transparent substance which is discharged intothe central cavity (Biedermann), and it is probable that theyrepresent a stage in the formation of mucus. Numeroustransitional stages between these two varieties of epithelium occur, and it is quite certainthat the differences are dueto the age of the glands, andtheir different states of secre-tion. Changes in the formof the cells, however, are pro-duced to a certain extent bythe contraction of the of the epitheliumis a muscula
. The biology of the frog . ese granules swell up, and become con- 184 THE BIOLOGY OF THE FROG verted into a transparent substance which is discharged intothe central cavity (Biedermann), and it is probable that theyrepresent a stage in the formation of mucus. Numeroustransitional stages between these two varieties of epithelium occur, and it is quite certainthat the differences are dueto the age of the glands, andtheir different states of secre-tion. Changes in the formof the cells, however, are pro-duced to a certain extent bythe contraction of the of the epitheliumis a muscular coat composedof smooth muscle cells whichlie in a meridional outermost coat of thegland is formed by a layerof fibrous connective function of the musclecells is the expulsion of thesecretion of the gland. Theglands of the skin are in con-stant motion (Ascherson, En-gelmann), as may be seen byan examination of the glandsin the web of the foot. Theychange not only in size, butalso in form, being now. FUj. 47. — Cross section of the skinof the frog. D, dermis or cutis ; E,epidermis ; , blood vessel; ,cutaneous gland cut through thecenter; the same from oneside ; d, duct of gland ; , \, horizontal fibers of connec-tive tissue ; , outer or horny layerof the epidermis ; , Malpighianlayer of the epidermis ; pg, pigmentcells. (After Howes.) rounded and now wrinkledand angular. Contraction may be caused by stimulation ofthe skin with irritant solutions or by the electric poison glands are larger and less abundant than the IX THE SKIN 185 mucus glands, and less uniformly distributed over the sur-face of the body. They are more numerous on the dorsalside of the body and hind legs, and they are especiallyabundant, and unusually large, in the lateral dermal to Junius, they occur on all parts of the skin,although they may be comparatively scarce in certain situa-tions. Like the mucus glands they possess a m
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