26 PROTOPLASM AND THE CELL. 



already been mentioned on p. 11 and are in this way enabled 

 to effect a physiological division of labor. 



The variations in form and structure which thus appear are 

 endlessly diversified. Cells may assume almost any conceivable 

 form, and there are even cells (e.g., Amahs, or the colorless 

 corpuscles of the blood) which continually change their form 

 from moment to moment. The variations in structure may in- 

 volve any or all of the three characteristic parts of the typical 

 cell, being at the same time accompanied by variations of form. 

 It is easy to understand, therefore, how cells may vary endlessly 

 in appearance, while conforming more or less closely to the same 

 general type. 



Meanwhile the protoplasm itself undergoes extensive altera- 

 tion. Even in young cells, or in the germ-cell itself, it may 

 ,~~ - j contain an admixture of other substances, 



and these may entirely change their 

 character or (as is especially common in 

 plant-cells) may become more abun- 

 dant as the cell grows older, taking the 

 shape of fluid, solid, or even gaseous de- 

 posits. Common examples of such de- 

 posits are drops of water, oil, and resin 

 ~ n granules of pigment, starch, and solid 

 no. 15. (After Ranvier.)- P rotekl matt f re > ^"1 crystals of mineral 



Part of a single fibre of vol- Substances like Calcium OXalate, pllOS- 



rSrX! P'' ate a " (1 "r'-onate, an.l silica. Bub- 

 n, nucleus. (xToo.) hies of gas sometimes appear in the pro- 



toplasm, but this is exceptional. The living substance itself 

 often changes in appearance as the cells become differentiated. 

 The protoplasm of voluntary muscles (Fig. 15) is firm, clear, 

 non-granular, highly refractive, and arranged in alternating 

 bands or stripes of darker and lighter substance. In some cases 

 (e.g., the outer portions of the skin, or of a hair, as explained 

 in Chap. II) the modifications of the cell-substance becomes so- 

 groat that both its physical and chemical constitution are entirely 

 altered, and it is no longer protoplasm, but some form of lifeless 

 matter. 



Protoplasm in Action. We may now briefly consider proto- 

 plasm from the dynamical or physiological point of view. We 



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