8 THE PHENOMENA OF LIFE 



perform the same functions, grow and reproduce in their turn. This cycle 

 of life is being continually passed through. 



In more complicated structures than the ameba, the life of individual 

 protoplasmic cells is probably very short in comparison with that of the organ- 

 ism they compose; and their constant decay and death necessitate constant 

 reproduction. The mode in which this takes place has long been the sub- 

 ject of controversy. 



It is now very generally believed that every cell is descended from some 

 pre-existing mother cell. This derivation of cells from cells takes place by 

 gemmation, which essentially consists in the budding off and separation of 

 a portion of the parent cell; or by fission or division. 



The exact manner of the division of cells is a matter of some difficulty, 

 and will not be described until the subject of the structure of protoplasmic 

 cells has been considered. 



STRUCTURE OF PROTOPLASM. 



Elemental Structure. Protoplasm was formerly thought to be homo- 

 geneous. It is now generally found to consist of the elemental divisions 

 called cells. Each cell, from a morphological point of view, consists of dif- 

 ferentiated parts, the most constant of which are the cell nucleus and the cell 

 cytoplasm. The cytoplasm is differentiated further into two substances, 

 spongioplasm and hyaloplasm. The spongioplasm or reticulum forms a fine 



^ _ Membrane of nucleus. 



Cell membrane. 



Achromatic substance of 



Cell reticulum. Chromatic substance of 



nucleus. 



FIG. 8. Cell with its Reticulum Disposed Radially; from the intestinal epithelium of a 



worm. (Carnoy.) 



network, increases in relative amount as the cell grows older, and has an 

 affinity for staining reagents. The hyaloplasm is less refractile, elastic, or 

 extensile, and has little or no affinity -for stains; it predominates in young cells, 

 is thought to be fluid, and fills the interspaces of the reticulum. The nodal 

 points of the reticulum, with the granular microsomes, found in the proto- 

 plasm, cause the granular appearance. 



The arrangement of the reticulum varies considerably in different cells, 

 and even in different parts of the same cell. Sometimes, for example, figure 



