THE CHEMISTRY AND PHYSICS OF THE CELL 57 



are found in living tissues seem to have a decidedly semiperme- 

 able character. 



From what we know at the present time of intracellular 

 physics and chemistry there is no necessity for assuming that 

 sernipermeable septa exist within the cell. All the intracellular 

 processes with which we are familiar could go on without such 

 structures. It is not necessary to assume a compartment 

 structure to explain the possibility of different chemical 

 reactions going on in different parts of the cell at the same 

 time, for most of the cell reactions seem to depend on enzymes, 

 which we know are not readily diffusible in solutions of colloids, 

 and, therefore, might remain fixed without requiring any 

 enclosing walls or retaining framework. Certainly, many cells 

 are free from structural cytoplasm, for we see particles of solid 

 matter moving about within them quite freely. In some cells 

 the nuclei migrate about in the cell, as also do digestive and 

 excretory vacuoles, which motion would seem to be rather 

 destructive if the protoplasm had a structure at all permanent. 



When a portion of the cytoplasm is cut free from the body of 

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

 ble fluid would do in another of different surface tension, and 

 not at all as if it were bound into a fixed structure by a frame- 

 work. 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 clas- 

 sical 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 ameboid 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 way that an 

 ameba would. 



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 inorganic salts seem to be, at least in part, contained in 

 the cells in chemical combination rather than in simple solution 

 in the water of the cell. There is much evidence indicating 



