18 THE CHEMISTRY AXD PHYSICS OF THE CELL 



found in plants, in which tlie ])r()niinent feature is tlie 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 w^alls 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, 

 may 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 differences 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 quantities usuall}^ not greatly dissimilar; hence they are as- 

 sumed to be the most important constituents of protoplasm, and are 

 sometimes called the primary constituents of the cell. ]\Iany other 

 secondary constituents may also be present, some of which are so 

 nearly universal that we are not sure but that they really are primary 

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

 tics 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 existing between different tis- 

 sues 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 

 pi'odiK'ts of cell activity. 



Protoplasm, as the term is generally used, includes the various 

 primary constituents with the fluids permeating or dissolving 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. ])articularly because of the confusion as to 

 whether it includes the nucleus or not, difrerent authors difVering in 

 this i'es|)ect in their usage of the word. 



CHEMISTRY OF THE ESSENTIAL CELL CONSTITUENTS 



To enuinei'ate the primary or essential constituents of tlu^ cell al)- 

 solntely is not ])ossil)le, for the rapid advances in cheiiiisti'v nuiy 



