22 GENERAL PHYSIOLOGY 



display a clearly recognizable nucleus, they nevertheless contain nu- 

 -clear material which, in accordance with Biitschli,^ appears in many 

 cases merely as dust-Hke fragments scattered through the cytoplasm. 

 At best, therefore, a cell can only be defined as a globule of protoplasm 

 containing a certain amount of nuclear material. 



The term protoplasm (protos, first; plasma, form) is usually em- 

 ployed as a synonym for living substance. Huxley, for example, 

 speaks of it as the physical basis of life, just as the cell has been desig- 

 nated by Briicke^ as the elementay functional unit. It should be 

 emphasized, however, that protoplasm is not a single substance, but 

 is composed of several. It is a definite chemical compound which, in 

 accordance with the histologists, possesses certain staining powers and, 

 in accordance with the physiologists, exhibits a certain behavior to- 

 ward the conditions under which it is made to live. In the second 

 place, it must be remembered that protoplasm differs somewhat in its 

 chemical composition and physical arrangement. Thus, the proto- 

 plasm composing the muscle cell is not at all identical with that form- 

 ing the cells of the liver or kidney or other organs. We know this to 

 be true, because the reactions of these diverse types of protoplasm are 

 not absolutely the same, but vary in accordance with their function. 

 And besides, even a single cellular unit most commonly contains more 

 than one kind of protoplasm, namely, the fundamental substance plus 

 certain adjuncts which to all appearances give rise to a division of 

 labor. Thus, it is conceivable that in single protoplasmic entities, 

 such as are presented by ameba, stentor and other unicellular organ- 

 isms, a certain portion of the substance is set aside to serve the pur- 

 pose of digestion, another that of excretion and still another that of 

 locomotion. 



The Structure of the Cell. — It is evident, therefore, that living 

 matter appears in the form of cells and that these cells may be either 

 single free-living organisms or may be combined into colonies to form 

 the tissues and organs of the more complex animals and plants. In 

 either case, whether forming a unicellular entity or united with others 

 into a multicellular organism, the cell presents certain morphological 

 and functional characteristics. Its form differs greatly, and while the 

 large majority of cells retain their shape throughout their life, a cer- 

 tain number of them, such as the ameba, change it constantly. It 

 may be taken for granted, however, that their fundamental shape is 

 round, or nearly so, and that almost any polyhedral form may be im- 

 parted to them by grouping them into tissues and organs. Moreover, 

 while some of them may attain an unusual length, others are equip- 

 ped with appendages in the form of pseudopodia, flagella and cilia. 

 Their size, on the other hand, differs only within relatively narrow 

 limits. By far the greatest number of them remain below the range 

 of ordinary vision and very few attain dimensions that may be 



^ tJber den Bander Bakterien und verw. Organismen, Leipzig, 1890. 

 * Sitzungsber. der Wiener Akad. der Wissensch., xliv, 1861. 



