44 THE CHEMISTRY AND PHYSICS OF THE CELL 



certain cells it at once forms a round drop, just as any insoluble 

 fluid would do in another of different surface tension, and not at all 

 as if it were bound into a fixed structure by a framework. Other 

 cells, however, retain their form under the same conditions. The 

 structure of even so evidently complicated a cytoplasm as that of 

 striated muscle fibers is in doubt; a classical observation on this point 

 is the passage of a minute worm through the substance of a muscle- 

 cell, its progress being as unimpeded as if there were no such things 

 as disks, bands, rods, and striae in the cell. Many features of ame- 

 boid movement also seem to indicate that the cytoplasm follows 

 much the same laws as a drop of fluid in a heterogeneous medium, for 

 we can make a drop of mercury or of chloroform in water, or of oil 

 in weak alcohol, react to various stimuli in much the same waj' that 

 an ameba would. If we look upon the cytoplasm as a drop of emulsion 

 colloid, the surfaces of the particles in the emulsion furnish of them- 

 selves adequate explanation of many of the phenomena of isolation 

 of chemical reactions, etc., without lacking in harmony with the evi- 

 dences of structural homogeneity. This hypothesis fits all sides of 

 the problem and has many supporters at the present time.*^ 



The question of structure in the nucleus is quite a different matter, 

 in so far as the chromatin threads and the nucleolus are concerned. In 

 ameboid movement the nucleus seems to play a passive role and to 

 be dragged about by the cytoplasm, indicating quite a high degree of 

 rigidity. It is probable, however, that the achromatic portion between 

 the chromatin threads and granules has much the same structure or 

 lack of structure as the cytoplasm. 



The various secretory granules, fat-droplets, pigment-granules, glycogen gran- 

 ules, keratin, etc., that may lie in the cytoplasm, are inconstant constituents, 

 varying with different cells, and under varying conditions in the same cells, and 

 lie beyond the scope of our discussion of the general composition of the cell. Ac- 

 cording to Ruzicka'*^ there is contained in all cells, both in nucleus and cytoplasm, 

 an insoluble substance which corresponds structurally to the "plastin" of the 

 cytologists, and chemically is related to the reticulins and other albuminoids; 

 this he looks upon as the ground substance of the cells, corresponding to the albu- 

 minoid ground substance or stroma of organized tissues. 



Certain of the granulations observed in the cj^toplasm of cells seem to be de- 

 finite, constant structures of the living protoplasm, and these are now called mito- 

 chondria, which term includes many forms of granules described \mder various 

 names." Their solubility and staining reactions suggest that they contain phos- 

 pholipins, perhaps associated with proteins. Their functional importance is in- 

 dicated by the fact that usually their number varies directly with the metabolic 

 activity of the cells, and they may lie related to histogenesis. 



Other histological cellular structures also permit of more or less satisfactory 

 identification by microchemical methods, and Unna""' especially has contributed 

 to this field. By staining sections with dyes of varying reaction, after extracting 

 the sections with various solvents, he has obtained evidence of the chemical nature 



"An excellent discussion of this question is given bv Alslierg, Science, 1911 

 (34), 97. 



** Arch. f. Zellforsch., 1908 (1), 5S7. 



« Sec review bv Cowdry, Amer. .lour. Anat., 191() (19), 423; Carnegie Inst. 

 Publ., No. 2."), 1918. 



" See review by Gans, Dcut. med.Woch., 1913 (39), 1944. 



