18 INTRODUCTION 



As applied to the animal tissues, the term " cell " is entirely 

 a misnomer, for it describes accurately only such forms of 

 " cells " as are found in plants, in which the prominent feature 

 is the limiting wall, forming a cell to enclose a fluid content. 

 In most instances the " cell " answers better to the definition, 

 " a mass of protoplasm "; but usage makes language, and no 

 possible confusion can arise from the prevailing, universal use 

 of the original term, except, perhaps, that the term is prone to 

 carry with it the thought of the walls of the cell being much 

 more prominent than they really are. This is not so unfortunate 

 a result, perhaps, for, as we shall see later, the limiting surfaces 

 of the cell, even when too thin to be readily demonstrable, 

 play a much more important part in cell chemistry than their 

 appearance indicates. 



The morphological division of the cell into cell wall, cytoplasm, 

 nucleus, and nucleolus can hardly be followed out chemically, 

 for if we surmount to some extent the difficulties in the way of 

 studying the different portions separately, we find that the dif- 

 ferences between them are rather quantitative than qualitative. 

 And, furthermore, however different the cells of one organ or 

 tissue may appear from those of another organ or tissue under 

 the microscope, when analyzed by the chemical methods at 

 present at our disposal we find the differences very slight indeed. 

 Certain substances are found in every living cell, and in quan- 

 tities usually not greatly dissimilar ; hence they are assumed to 

 be the most important constituents of protoplasm, and are 

 sometimes called the primary constituents of the cell. Many 

 other secondary constituents may also be present, some of 

 which are so nearly universal that we are not sure but that 

 better methods would show them to be constant and primary 

 cell components ; such are fat and glycogen. Others are charac- 

 teristics of certain cells, such as melanin and keratin, or specific 

 products of cell metabolism, such as mucin and the specific 

 enzymes. The great histological and chemical differences ex- 

 isting between different tissues depend often on these secondary 

 products, as in fat tissue and squamous epithelium ; or upon 

 the intercellular substance, as with connective tissue, cartilage, 

 bone, etc., which may be looked upon as products of cell activity. 



Protoplasm, as the term is generally used, includes the 

 various primary constituents with the fluids permeating or dis- 

 solving them, but does not include the more conspicuous 

 secondary constituents, such as fat droplets, pigment granules, 

 etc., nor the cell membrane when such exists. Evidently it is 

 a very indefinite term, to be avoided as much as possible, par- 



