THE CELL 15 



a cell membrane and containing a spherical body, the nucleus, 

 enclosed in its own membrane. The body of the cell, or cytosome, 

 includes all the protoplasm outside the nucleus, both nucleus and 

 cytosome being composed of protoplasm whose composition diifers 

 in each case. 



Protoplasm. — Protoplasm is chemically a complex mixture of 

 l)r()teins and their derivatives, carbohydrates, lipoids, and inor- 

 ganic salts associated with a large amount of water. Physi- 

 cally, jirotoplasm has the properties of a complex colloidal system, 

 capable of those changes in viscosity which are due to revers- 

 ible gels occurring in the living cell. Living cells are visible as 

 pale, colorless, homogeneous masses in which, with proper illu- 

 mination, various refractive structures may be observed. The 

 cell membrane and the nuclear membrane may appear brighter 

 because of the higher refracti\ity of their components. Likewise 

 the chromosomes of the nucleus, together with some of the formed 

 granular or fibrillar elements of the cytosome, may stand out. For 

 the most part, our knowledge must depend upon examination of 

 fixed and stained material in which the various elements of the 

 protoplasm are precipitated as insoluble substances which are then 

 stained with various dyes. The precipitation process also preserves 

 the membranes; and, depending upon the reagents used, other 

 structures of the cytosome, such as the mitochondria, or chondrio- 

 somes, Golgi apparatus, and centrioles, may be precipitated for 

 staining and study. (Fig, 1.) 



Cytosome.— The appearance of the cytoplasm in fixed materials 

 varies with the method used in preparation, and also with the type 

 and physiological state of the cell. It was this variability with 

 fixatives which caused much of the early disagreement and debate 

 among scientists as to the nature of protoplasm, since each based 

 his conception on his prepared material. Accordingly, some investi- 

 gators held that protoplasm was essentially granular; others believed 

 it to be alveolar and to resemble an emulsion; still others held that 

 it had a reticulum of slender fibrils associated with fine granules. 

 Actually all these conditions may be observed in the same type of 

 cell, if it be treated with various techniques. However, certain 

 definite structures or organelles are difterentiated in the cytosome. 

 Chondriosomes, or mitochondria, are elements demonstrated 

 within the cytoplasm after certain techniques. They are quite 

 universal in occurrence, though variable in shape. Usually they 

 are granular in young or embryonic cells but filamentous or rod- 



