THE APPEARANCE OF PROTOPLASM. 



the microscope (500-1000 diameters) it appears to consist mainly 

 of two constituents, viz. : (a) a perfectly clear semi-tiuid basis, (b) 

 minute, solid, highly refracting granules embedded in the clear 

 substance. AVith the most perfect and powerful lenses skilled 

 observers detect also fine threads or fibres in the clear substance, 

 but these elude all ordinary observation. In most cases part of 

 the protoplasm of the cell appears as a definite, rounded mass 

 known as the nucleus, which is easily distinguishable from the 

 remaining protoplasm by its darker appearance (due to its greater 

 refractive power) and the deeper color which it assumes when 

 treated with solutions of coloring mat- 

 ter, la spite of this apparently simple 

 structure, we must infer from its mar- 

 vellous powers that protoplasm is really 

 endowed with a molecular constitution 

 of extreme complexity, probably far 

 exceeding that of any lifeless sub- 

 stance. 



Such protoplasm, simple and un- 

 modified, is seldom found absolutely 

 pure even in very young cells. It con- 

 tains almost alwaj'S an admixture of 

 other substances which become more 

 abundant as the protoplasm grows older, 

 taking the shape of fluid, solid or even 

 gaseous deposits. Common examples 

 of such deposits are drops of water, 

 oil, and resin, granules of pigment, 

 starch and solid proteid matters, and crystals of mineral substances 

 like calcium oxalate, phosphate and carbonate, and silica. Bub- 

 bles of gas are said to appear rarely in the protoplasm ; but this, 

 if true, is certainly exceptional. The living substance itself often 

 changes in appearance as the cells become differentiated. The 

 protoplasm of voluntary muscles (Fig. 13) is firm, clear, non-granu- 

 lar, 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 great that both its 

 physical and chemical constitution are entirely altered, and it is 

 no longer protoplasm, but some form of lifeless matter, 



FIG. 14. Change of form in a 

 contracting muscle. A , muscle 

 in the ordinary or extended 

 state ; B, the same muscle when 

 contracted. (Diagram). 



