i io HISTORY OF THE HUMAN BODY 



into one another in such a way as to make the classification 

 indeterminate. 



Glands may be also divided according to their method of 

 furnishing the secretion, since some cells, when surcharged, 

 liberate their fluid by bursting, and thus become destroyed, 

 while others allow their secretion to pass through their walls, 

 retaining their physiological life for an indefinite period. In the 

 former case the supply of cells is kept up by a constant prolifer- 

 ation from a zone of growth ; in the other case the community 

 of cells retains its identity. The glands in the former case are 

 termed necrobiotic, in the other they are vitally secretory. 

 This physiological distinction is often of use in determining 

 homologies at times when the structure is non-committal or 

 misleading. 



Unlike most other structures, the integumental glands 

 of vertebrates do not appear to have a continuous history 

 in the various Classes, but are developed in each Class, or 

 even in specific cases, to suit the needs of particular environ- 

 ment or habits. In fishes and amphibians the main function 

 of integumental glands is to secrete a protective slime, to 

 defend the surface from the action of the water, to which, as a 

 secondary function, probably accidental at first, there is often 

 added to the secretion some acrid or even actively poisonous 

 quality, for defense against predaceous animals. The glands 

 supplying this function are often of the unicellular type, with 

 a narrowed neck at the surface, and called beaker cells from 

 their shape; the simple acinous type, in the form of flask- 

 shaped glands, is widely distributed among amphibians, where 

 the glands often occur in clusters, causing a conspicuous pro- 

 tuberance. The integument of the Sauropsida is characterized 

 by an almost complete absence of glands, certain special ones 

 appearing in definite localities and employed for some special 

 purpose. Such are the cloacal glands of snakes, which secrete 

 for defensive purposes a milky fluid having a nauseating odor, 

 and the musk glands of certain turtles, which may be defensive 

 or used as a sexual allurement. In the males of certain lizards 

 a single line of glands opens along a definite row of scales on 



