GENERAL PHYSIOLOGY 169 



with the environmental salts. If a given organism lives in a 

 hypertonic merlium, water formed as a waste product of metabolism 

 does not accumulate but passes out by exosmosis as in marine forms. 

 Some types of Protozoa, furthermore, may be transferred from fresh 

 water to salt without fatal results. Thus Zuelzer (1903) has shown 

 that Amoeba vcrnwosa upon transference from fresh water to salt 

 continues to live. It not only loses its contractile vacuole but the 

 protoplasm becomes much condensed, evidently through loss of 

 water. Here difference in density of the surrounding medium is 

 largely responsible for loss of the organ characteristic of fresh-water 

 forms, but changes in permeability of the cell membrane due to 

 salts in the new medium undoubtedly play an important part. 

 Other experiments by difi'erent observers bear out the same prin- 

 ciple. Thus dilution of the normal neutral salts in the medium 

 causes enlargement of the contractile vacuoles in Euglena and in 

 ciliates according to Klebs (1893) and Massart (1891), while 

 increased concentration leads to reduction in size, retardation in rate 

 of contraction, or total disapj)earance of the vacuole. 



While there is justification for Hartog's view of the purely 

 physical significance of the vacuole, there is every reason for believ- 

 ing that water in protoplasm picks up any soluble waste matter that 

 may be present, and holds it in solution. Early experiments to prove 

 this, by Brandt (1885), Griffiths (1889), and others using chemical 

 indicators, or the murexid test for uric acid, were not convincing, 

 and the function of the contractile vacuole as a primitive type of 

 excretor\' organ remained an hypothesis. 



Not only water, CO2 (see Lund, 1918) and urea, but other pro- 

 ducts of metabolism as well, are frequently found in the protoplasm 

 of different Protozoa. These are usually present in crystalline form 

 or in amorphous heaps, which are rather loosely spoken of as 

 "excretory stuffs" without evidence as to their origin and signifi- 

 cance. The crystals often seen in Paramecium were identified by 

 Schew^iakoff (1893) as calcium phosphate combined with some 

 organic substance. Similar crystals have been described by 

 Schaudinn, Schubotz and others from the protoplasm of different 

 kinds of Protozoa. Schewiakoff" found that the crystals of Para- 

 mecium are not defecated as are undigested food substances, but are 

 first dissolved and then disposed of— presumably with the water of 

 the contractile vacuoles. 



The function of the contractile vacuole in Protozoa thus has long 

 been a disputed problem. The views of the older students of the 

 group, with their conceptions of structural complexity of these uni- 

 cellular organisms, fantastic today^ nevertheless have a certain his- 

 torical interest. The idea that a vacuole is a rudimentary beating 

 heart as interpreted by Lieberkiihn (185(3), Claparede and Lachmann 

 (1854 and 1859), Siebold (1854) and Pritchard (1801) was no less 



