8 



THE PHENOMENA OF LIFE 



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 

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

 called cells. Each cell, from a morphological point of vi 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 

 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 



Cell membrane 

 Cell reticulum .... 



Membrane of nucleus. 



Achromatic substance of 



nucleus. 

 Chromatic substance of 



nucleus. 



PlO. 8. Cell with its Ketiailum Disposal K;u!i;illv; fn.ni Mir intrst.innl epithelium of n 



worm. (Carnoy.) 



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 8, 

 the. meshwork has an elongated radial arrangement from the nucleus; at 

 others, the meshwork is more evenly disposed, as in figure 9. At the junctions 

 of the fibrils there are usually slight enlargements or nodes. 



In some cells, particularly in plants, but also in some animal cells, there is 

 a tendency toward a formation of a firmer external envelope, constituting in 

 vegetable cells a membrane distinct from the more central and more fluid part 

 of the protoplasm. In such cases the reticulum at the periphery of the cell is 

 made up of very fine meshes. The membrane when formed is usually pierced 

 with pores by which fluid may pass in, or through which protrusion of the 

 protoplasmic filaments forming the cell's connection with other cells surround- 

 ing it may take place. 



