442 THE CONTRACTILE VACUOLE 



possessing more or less permanent vacuole systems, these organelles ap- 

 pear to be surrounded by morphological membranes. 



3. Direct evidence concerning the function of contractile vacuoles is 

 almost entirely lacking. Indirect evidence indicates that in fresh-water 

 forms the vacuole protects the organism against excessive dilution of its 

 cytoplasm. In marine and parasitic forms such a function would seem to 

 be largely superfluous, although even in these the elimination of at 

 least a small quantity of water by some mechanism appears to be neces- 

 sary. Direct evidence indicating the presence of waste products of me- 

 tabolism in the vacuolar fluid is very scant, although, in those forms 

 possessing relatively impermeable surface structures, the vacuole is the 

 only visible means by which such wastes may be passed to the exterior. 



4. In some Protozoa three types of cytoplasmic inclusions have been 

 identified, in others only two types. In all Protozoa so far examined with 

 this in view, at least some of the inclusions are osmiophilic. In some oth- 

 ers these osmiophilic inclusions are also stainable by neutral red, but not 

 by Janus green. Osmication of certain Protozoa by one technique or an- 

 other frequently shows more than one type of inclusion to be osmiophilic, 

 but generally one of these resists bleaching by hydrogen peroxide or 

 turpentine more completely than the others. Such inclusions are generally 

 recognized as the Golgi apparatus. By comparing living organisms with 

 those stained vitally with various dyes, as well as with others impregnated 

 with osmium or silver, identity of the Golgi apparatus and the cyto- 

 plasmic inclusions concerned with the origin of the contractile vac'iole 

 has been established for many forms. The usual form of protozoan Golgi 

 apparatus is granular, globular, or rod-like. In a few species {Para- 

 mecium, Dogielella, Chilodon, and others), it frequently appears as a 

 network, while in others [Lionotus, Nassula, Campanella, and others) 

 it is in the form of a thick ring, or membrane, surrounding part or all 

 of the vacuole. Evidence has been presented which indicates that in some 

 of these, if not all, a granular structure has been overimpregnated, this 

 causing it to assume the appearances mentioned. It therefore appears that 

 fluid which is expelled from the organism by the contractile vacuole 

 originates as droplets in association with the Golgi apparatus, although 

 the Golgi apparatus is not necessarily in intimate contact with the vacu- 

 ole. Concerning the origin of secretions in metazoan gland cells, Bowen 

 (1929, p. 511) states: 



