CH. II.] PROTOPLASM. 9 



In order to study the microscopic structure of such transparent objects as 

 cells, it is necessary to have recourse to various methods of fixing and staining. 

 When one sees certain appearances after such treatment of the cells, the 

 question arises whether they may not be due to the action of the reagents 

 employed. Appearances which are undoubtedly produced artificially in this 

 way are generally spoken of as artifuctx. The network just described is 

 regarded by some observers as an artifact, but it is impossible at present to 

 state this positively. Hardy, in particular, has shown that a film of any 

 colloidal substance like gelatin will, when it sets, present the appearance of 

 a network, and he regards it as probable that the network seen iu cells may 

 be due to a similar setting or coagulation of the protoplasm which occurs 

 either when the cell dies, or is fixed by hardening reagents. 



The granules in protoplasm are partly thickened portions of the 

 spongioplasm, but in addition to this there appear to be free 

 granules, some fatty in nature (staining black with osmic acid), 

 some composed of the substance called glycogen or animal starch 

 (staining reddish-brown with iodine), and sometimes in a few 

 unicellular animals they consist of inorganic (calcareous) 

 matter. But by far the most constant and abundant of the 

 granules are like the main substance of the protoplasm, proteid 

 or albuminous in composition. 



The chemical structure of protoplasm can only be investi- 

 gated after the protoplasm has been killed. The substances it 

 yields are (i) Water; protoplasm is semifluid, and at least 

 three-quarters of its weight, often more, are due to water. 



(2) Proteids. These are the most constant and abundant of 

 the solids. A proteid or albuminous substance consists of carbon, 

 hydrogen, nitrogen, oxygen, with sulphur and phosphorus in 

 small quantities only. In nuclein, a proteid-like substance 

 found in the nuclei of cells, phosphorus is more abundant. 

 The proteid obtained in greatest abundance in the cell proto- 

 plasm is called a nucleo-proteid, that is to say, it is a compound 

 containing varying amounts of this material nuclein with proteid. 

 White of egg is a familiar instance of an albuminous substance 

 or proteid, and the fact (which is also familiar) that this sets 

 into a solid on boiling will serve as a reminder that the greater 

 number of the proteids found in natxire have a similar tendency 

 to coagulate under the influence of heat and other agencies. 



(3) Various other substances occur in smaller proportions, the most 

 constant of which are lecithin, a phosphoriscd fat ; cholesterin, 

 a monatomic alcohol ; and inorganic salts, especially phosphates 

 :in<l chlorides of calcium, sodium, and potassium. 



The large quantity of water present should be particularly 

 noted ; the student when first shown diagrams of the reticulum 

 in protoplasm is apt to imagine that it consists of a firm solid, 

 like a system of wires pervading a jelly. The reticulum is only 

 slightly more solid than the hyaloplasm. 



