THE VITAL PHENOMENA OF CELLS 



41 



free-living cell body in a manner exactly the reverse of their ingestion. The 

 remnant is brought to the surface of the cell, the protoplasm gives way in 

 this particular place, the body to be 

 eliminated is extruded, and the cell 

 withdraws from it. 



The gaseous excretions, carbon diox- 

 ide and oxygen, are probably eliminated 

 according to the laws of diffusion (cf. 

 Chapter IX). 



We have as yet but little reliable in- 

 formation as to how dissolved substances 

 leave the cell. To reason by analogy 

 with the manner of their absorption we 

 ought to find at work besides the osmotic 

 processes, an active influence on the part 

 of the cell itself. If the products can- 

 not be removed immediately they are 

 sometimes rendered harmless by a com- 

 bination of some sort. For example, ox- 

 alic acid, which is poisonous, is bound 

 up with calcium in the insoluble and 

 therefore harmless compound calcium 

 oxalate. In Chapter XIII we shall dis- 

 cuss a number of analogous processes in 

 the higher animals. A highly special- 

 ized mode of removing fluid waste from 

 the body of an Infusorian is shown in 

 Fig. 24. 



n 



FIG. 24. Frontana leucas, after Schewia- 

 koff. N, macronucleus ; n, micronu- 

 cleus; c, one of several excretory canals 

 leading into the excretory vacuole; v, 

 by contraction of which the fluid contents 

 are ejected. Several ingested diatoms are 



to be seen. 



F. SECRETION 



In unicellular as well as in multicel- 

 lular organisms, though in the latter 

 probably to a much greater extent, the 



cells by their own activity form various substances which either serve the 

 purposes of the cell itself, or, in the multicellular forms, are essential to 

 the purposes of the entire body. We include all such substances under the 

 head of Secretions. 



To the secretions belong the enzymes and analogous compounds, such as the 

 products of the so-called internal secretions (cf. Chapter XI), also most of 

 the skeletal substances, as the chitinous cases of insects, the calcareous shells 

 of Foraminifera, the cell membranes, etc.; and, finally, the intercellular sub- 

 stances such as occur in the fibrillar connective tissue, cartilage and bone. How 

 far these substances arise by transformation of the living protoplasm, or are 

 only produced by its activity cannot be regarded as settled. But it is probable 

 that the latter alternative holds in the case of those structures which are formed 

 of silicic acid or calcium carbonate or phosphate; for it can scarcely be sup- 

 posed that structures of this kind arise directly from protoplasm. 



