PROTOPLASM OF PROTOZOA 71 



divide several times, forming a number of vacuoles. In other cases food 

 vacuoles have been observed to fuse (Mast and Hahnert, 1935). 



From the comparatively few accounts of the physical structure of the 

 food vacuole membrane, it is not possible to give an analysis of its struc- 

 ture. However, it seems reasonable to conclude that it is usually of mo- 

 lecular dimensions, capable of resisting deformation, and that it is 

 permeable to enzymes, water, certain dyes, and digested food materials. 

 Its architecture is probably much the same as that of the surface mem- 

 brane, except perhaps for its thickness. 



5. Other types of vacuoles. — In addition to the contractile vacuolar 

 system and the food vacuoles, many other types of vacuoles may be found 

 in the cytoplasm of the Protozoa. Such vacuoles may be characteristic 

 of the cytoplasm, as the crystal vacuoles in Amoeba (Mast and Doyle, 

 1935a), transitory vacuoles (Hopkins, 1938), large acid-filled vacuoles 

 associated with changes in specific gravity of Noctiluca (E. B. Harvey, 

 1917; Lund and Logan, 1925); or they may be induced by certain ex- 

 perimental methods, such as change in the salt content of the environ- 

 ment (Schaeifer, 1926), exposure to dyestuffs. X-rays, poisons, and so 

 forth (Heilbrunn, 1928). These vacuoles, too, frequently have been 

 reported to fuse; normally as in the coalescence of vacuoles in Noctiluca 

 (Lund and Logan, 1925), and experimentally in Paramecium in which 

 King and Beams (1937) observed the crystal vacuoles to fuse when cen- 

 trifuged. 



Little is known concerning the actual physical structure of the mem- 

 branes of such vacuoles, but there is no reason for believing that their 

 structure differs greatly from that of other surface membranes surround- 

 ing vacuoles, such, for instance, as the feeding vesicles of the contractile 

 vacuole of a form like Euplotes or Varamecium. 



ADHESIVENESS OR STICKINESS 



Among the physicochemical properties of the protoplasm-liquid 

 medium interface, adhesiveness is of importance in considering such 

 subjects as ameboid movement, tissue culture, leucocyte activity, im- 

 munity, and cell movements in embryology. Pfeiffer (1935) has listed 

 the literature on adhesiveness, which is scattered very widely. 



It is well known that amoebae creep on vertical surfaces, even on the 



