42 BIOLOGY OF THE PROTOZOA 



istic of the inner plasm. Spherical spaces or vacuoles are also visible 

 in the living forms, sometimes with solid, usually foreign, matter 

 within them (gastric vacuoles, defecatory vacuoles), sometimes 

 filled with clear watery fluid (contractile vacuoles) which is emptied 

 to the outside at regular intervals, or sometimes filled with fluids 

 which are not discharged (stationary vacuoles, or cavulae of Wetzell). 

 The same form, when fixed with a good killing agent, and properly 

 stained, gives a permanent picture of the granules, vacuoles and 

 other cell parts as they were at the instant of fixation. The nucleus 

 now stands out as the most conspicuous part of the cell, while the 

 granules are seen to be of different sizes and to react differently 

 after treatment with different stains. 



In most cases the finer physical structure of the protoplasm can be 

 seen both in the living cell and after fixation. It is best described 

 as a foam structure similar to the bubbles of soap suds but with 

 "bubbles" or alveoli of microscopic size. Imagining an optical 

 section through soap suds in which granules of finely-powdered 

 carmine have been distributed by stirring, the picture presented 

 would be a network or meshwork of water, soap and carmine, and 

 with an accumulation of carmine granules where three planes of 

 contiguous bubbles come together, while the spaces within the 

 meshes would be filled with air. The apparent network, however, 

 is merely the optical section of continuous walls of bubbles enclosed 

 on all sides by the water and soap. The physical structure of the 

 protoplasm of a few Protozoa, called spumoid structure by Rhumbler, 

 may be accurately compared with such an emulsion of soap and 

 water. An analogous network, usually of exquisite fineness, repre- 

 sents the more solid substance of protoplasm; the apparent fibers 

 forming the meshwork in some cases at least are the optical sections 

 of continuous walls, which, like the soap bubbles, enclose materials 

 of lesser density. Butschli who, with Rhumbler, has studied the 

 finer structure of protoplasm of lower plants and animals as well 

 as that of higher forms, was the first to compare such structures 

 with the alveolar structure of emulsions like soap and water, oils 

 and water, etc. The granules of protoplasm, corresponding in posi- 

 tion with the carmine of the soap suds, lie in the substance of the 

 denser network of interalveolar material to which Doflein applied 

 the term stereoplasm. The alveolar substance, called rheoplasm by 

 Doflein, corresponds in position with the air of the soap bubbles. 



All who have investigated protoplasm agree that it is not a homo- 

 geneous substance but a mixture of colloidal substances in the 

 physical state described by Ostwald as an emulsoid in which the 

 interalveolar materials act in the manner of a dispersing agent 

 while the more fluid intra-alveolar substances are dispersed, but all 

 arc subject to reversal of phase. 



While the alveolar structure of protoplasm is convincingly demon- 



