THE LIVING SUBSTANCE 5 



(matrix) of a very fine emulsion (Fig. 121). Nearly all cells or 

 organized droplets of protoplasm possess certain parts which are 

 typical of them. Chief among these is the nucleus (Fig. 4). 

 It is (usually) a large, central body of vital importance to the cell. 



When speaking of protoplasm it is customary to include the 

 nucleus and to use the word cytoplasm when the nucleus is to be 

 excluded, though actually few biologists bother always to make 

 the distinction. What has been said of protoplasm in the pre- 

 ceding paragraphs refers especially to the cytoplasm, though 

 the properties of the nucleus are similar. It, too, is of a slimy, 

 elastic, sometimes quite thin, sometimes thick material which is 

 physically much like cytoplasm but differs from it in chemical 

 constitution. It exists as a distinct globule within the cyto- 

 plasm, influenced by the latter just as it in turn influences, if it 

 does not to some extent control, the cytoplasm. The nucleus is 

 separated from the cytoplasm by a distinct nuclear membrane, 

 which breaks down during cell division when there results a more 

 intimate contact between cytoplasm and nuclear matter. During 

 the time that the two substances are thus intimately associated, 

 there is probably a marked influence of the one on the other. 

 Possibly at this time cytoplasmic characteristics are transferred 

 to the nucleoplasm and thus transmitted as hereditary traits. 



Protoplasm usually appears to be quiet except for occasional 

 minor disturbances, but there are times when it is exceedingly 

 active, its activity taking the form of rapid flowing. Streaming 

 protoplasm is a sight that never fails to hold the interest of a 

 "protoplasmologist." The flow may be circular, as it is within 

 a typical cell, or in only one direction at a time. The latter 

 one-way streaming is usually characteristic of cells that are 

 exceptional in form, such as the Plasmodium of a slime mold 

 (Fig. 1) or the long tubes of bread mold and certain algae {Vau- 

 cheria). Protoplasmic streaming is very typical of some cells 

 and is nearly always present in them (Amoeba and slime molds). 

 In other cells, streaming is seldom if ever to be seen, though some 

 movement of the protoplasm is likely to occur within every cell 

 from time to time. 



Protoplasm often develops superficial processes or growths 

 which become at times very striking. These surface processes 

 are formed, sometimes regularly and normally, when they may 

 serve a useful purpose, and sometimes exceptionally and appar- 



