2 INTRODUCTION TO CYTOLOGY 



increasing use of the methods and results of the physiologist, the physicist, 

 the chemist, the geneticist, and the taxonomist, as well as those of the 

 microscopist. The continued advance of this branch of biology will 

 depend largely upon the measure in which these various investigators 

 recognize the truly complementary nature of their tasks, and upon the 

 rigor with which their guiding theories are checked by sound observation. 



Description of the Cell. — The term cell was introduced in the seven- 

 teenth century by Robert Hooke and other microscopists, who applied it 

 to the small cavities in the honeycomb-like structure which they dis- 

 covered in plant tissues. Today the term denotes primarily the proto- 

 plasmic "cell contents," to which, strangely enough, the early workers 

 attached little importance. The term protoplast, proposed by Hanstein 

 (1880), is more appropriate and has come into fairly general use, but 

 long usage and brevity have insured the permanence of the older term. 

 The structural features of ordinary cells will now be sketched in their 

 barest outlines by way of introduction to the detailed descriptions and 

 discussions to follow in subsequent chapters. 



The three most constant constituents of the typical cell are the 

 cytoplasm, in which other cell organs and inclusions are imbedded, the 

 nucleus, bounded by its own membrane, and the plasma membrane differ- 

 entiated at the outer boundary of the cytoplasm.^ The cytoplasmic 

 portion of the cell may be referred to as the cytosome. 



The nucleus consists of a ground mass of hyaline fluid known as 

 karyolymph ("nuclear sap"), an imbedded reticulum composed of kary- 

 otin, which is usually highly stainable, one or more true nucleoli, or 

 plasmosomes, and a limiting nuclear membrane. Frequently masses of 

 karyotin known as chromocenters are present at certain points in the 

 reticulum (see Figs. 23, 24, and 73, 30). 



The cytoplasm, a more or less transparent, viscous fluid, may, with 

 its formed components and inclusions, occupy practically the entire 

 volume of the cell. This is generally true of animal cells. In most 

 plant cells a considerable amount of cell sap is present in one or more 

 vacuoles. Often it far exceeds the cytoplasm in volume, the cytoplasm 

 constituting a thin layer about it and lining the cell wall. In many cases 

 the cytoplasm forms a system of strands that often show active streaming. 

 Where it meets an enclosed vacuole it is limited by a vacuole membrane, or 

 tonoplast. Externally the cytoplasm is bounded by a specialized layer of 

 ultramicroscopic thinness, the plasma membrane. In some cells, for 

 example certain amcebse, a comparatively thick outer region known as the 



1 According to an older usage only the extra-nuclear portion of the protoplast was 

 called "protoplasm," which was unfortunate because of the fact that the nucleus also 

 is composed of protoplasm, or living substance in the broader sense. It is now the 

 general custom to avoid this ambiguity by employing Strasburger's terms cytoplasm 

 and nucleoplasm (kanjoplasm, Flemming). The older usage has not, however, been 

 entirely superseded. 



