ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 13 



and that further, it is impossible in a short phrase to give an 

 adequate definition of its meaning; and finally, that frequently it 

 is difficult to determine what part of the cell really consists of 

 protoplasm, and what does not; yet, in spite of all this, the 

 necessity of the conception remains. Similar objections could be 

 raised against a number of other words which we use for certain 

 definite compounds present in organic bodies. For instance, to 

 designate a certain portion of the nucleus we use the term nuclein 

 or chromatin, which is considered fairly adequate by many people. 

 And yet the microscopist is bound to admit that it is impossible to 

 state exactly which part of a resting nucleus consists of linin, and 

 which of nuclein, or to determine in any special case whether too 

 much or too little has been stained. 



Now the term protoplasm is quite as necessary in speaking 

 about the constituent parts of a cell. Only it must be stipulated 

 that the word protoplasm must not be understood to designate a 

 substance of definite chemical composition. 



The word protoplasm is a morphological term (the same is true in 

 a greater or less degree of the word nuclein, and of many others) ; 

 it is an expression for a complex substance, which exhibits a 

 variety of physical, chemical, and biological properties. Such ex- 

 pressions are absolutely necessary in the present state of our 

 knowledge. Any one who is acquainted with the history of the cell 

 knows what a number of observations and how much logical 

 thought were necessary before this conception was arrived at, and 

 further is quite aware that with the creation of this expression the 

 whole theory of cells and tissues gained in depth and significance. 

 How much wordy warfare was necessary before it was established 

 that the cell contents, and not the cell membrane, constitute the 

 essential portion of the cell, and further that amongst these cell 

 contents a peculiar substance is invariably present, which takes 

 part in the vital processes in quite a different way from the cell 

 sap, the starch granules, and the fat globules. 



Thus we see that the use of the word protoplasm is not only 

 justifiable from an historical point of view, but also from a 

 scientific one, and we will now proceed to endeavour to explain 

 what is meant by the term. 



6. General Characteristics of Protoplasm. The proto- 

 plasm of unicellular organisms, and of plant and animal cells (Figs. 

 1 and 2), appears as a viscid substance, which is almost always 

 colourless, which will not mix with water, and which, in con- 



