STRUCTURE OF THE CELL. 41 



in its living state, it often appears approximately homo- 

 geneous. Its substance is usually slightly fluid, but it may 

 be firm and compact in passive cells. It is usually transluc- 

 ent, but there are often obscuring granules of different kinds. 

 In many cases, its appearance suggests a fine emulsion; 

 indeed Butschli has succeeded in making marvellous imita- 

 tions of cells out of ingeniously contrived emulsions. 



In thinking of the cell-substance, we must distinguish 

 clearly between the genuinely living material or protoplasm, 

 of whose nature we know almost nothing, and associated 

 substances, such as starch, fat, or pigment, whose chemical 

 composition can be ascertained. The associated substances 

 which often crowd the protoplasm, are due to the chemical 

 ascent of food-material towards protoplasm, or to the chemical 

 disruption which protoplasm undergoes or produces as it lives. 



when a small portion of an animal is " fixed " or hardened 

 by some reagent like osmic, chromic, or picric acid, dehy- 

 drated by being soaked in various strengths of alcohol, 

 stained with carmine or some other dye, dehydrated again 

 in alcohol, soaked for hours in melted paraffin, cut in thin 

 sections with a razor or microtome, cleared from the 

 paraffin by immersion in turpentine, mounted in Canada 

 balsam or the like, and examined under the high power 

 microscope, details of cell structure can be seen which are 

 rarely recognisable in the fresh unprepared cells. The cell- 

 substance is far from being homogeneous ; it seems to consist 

 of a fine network with a more fluid material in the meshes. 

 In other cases, the structure is more like that of an emulsion, 

 with innumerable minute vacuoles. Within the cell-substance 

 of reproductive cells and some others, several recent in- 

 vestigators have described a "central corpuscle," quite apart 

 from the nucleus. It may be a " centre of force " within 

 the protoplasm. 



Many very valuable results in regard to the minute 

 structure of cells have been reached by histologists, but there 

 is sometimes a tendency to push the structural analysis 

 beyond the limit of physiological interest; nor is it always 

 remembered that the real structure of the cell may be 

 considerably affected in the course of that complex technique, 

 of which I have just given one of the simplest illustrations. 

 {b) The Nucleus. — Almost every cell contains a nucleus or 



