THE CELL AND THE TISSUES. 



THE VITAL MANIFESTATIONS OF THE CELL. 



The characteristics which distinguish the structural units of living 

 organisms from those of the inorganic world, may be conveniently 

 grouped as Vegetative, Metabolism, Growth, Reproduction; Ani- 

 mal, Irritability, Motion. 



Metabolism is that process by which the cell selects and assimi- 

 lates, from the surrounding food-materials, those substances adapted 

 to the particular needs for its nutrition and function, so changing 

 and incorporating into its own substance the materials so acquired 

 that they become an integral part of the cell. By a still further 

 exercise of this process the assimilated materials are converted into 

 new substances, which may be retained within the cell, or, as is 

 frequently the case, given up as the various secretions of the body. 



Growth, the natural sequence of assimilation, may affect the cell 

 equally in all parts, thereby producing a uniformly enlarged ele- 

 ment; such normal or typical increase is, as a rule, 'hindered by the 

 impression of neighboring elements, such limitations resulting in 

 many local alterations of form, as conspicuously seen in epithelial 

 tissues. It is, however, the principle of unequal growth that exerts 

 the greatest influence in producing specializations of form, as exam- 

 ples of which the cells of muscle, the crystalline lens, or connective 

 tissue are familiar. 



Reproduction, the culminating phenomenon of the life-history of 

 the cell, occurs by two modes : 



a. By direct division without karyokinesis. 



b. By indirect division with karyokinesis. 



Direct division, by which a cell rich in protoplasm, as the white, 

 blood-corpuscle, constricts, cuts off, and sets free a portion of itself, 

 while undoubtedly 



taking place in the FIG. 6. 



multiplication of the 

 simplest organisms, 



Or of the least dif- Direct cell-division of colorless blood-corpuscle. 



ferentiated elements 



of higher types, is no longer regarded, as formerly, as the most 

 important and usual mode of cell reproduction ; the observations 

 of the last decade have shown that its occurrence must be accepted 

 rather as exceptional than as customary. 



Indirect division, preceded by the complicated cycle of nuclear 

 changes collectively termed karyokinesis, is now recognized as 

 being the usual mode of the reproduction of cells of all kinds, in 

 pathological as well as in normal conditions. The recognition and 

 elucidation of these important phenomena have been largely due to 



