GENERAL PHYSIOLOGY 177 



nets of metabolism as well, are found in the protoplasm of differ- 

 ent 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 or significance. The 

 crystals often seen in Paramecium were identified by Schewiakoff 

 (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. Schewia- 

 koff found that the crystals of Paramecium 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 Lieberkuhn (1856), Claparede and Lachmann 

 (1854 and 1859), Siebold (1854) and Pritchard (1861) was no less 

 incongruous than the supposition of Ehrenberg (1838) that the 

 contractile vacuole is an organ connected with the gonadal system. 



With development of knowledge of structure and function of the 

 Protozoa, and particularly of the mechanism of vitality, more 

 reasonable hypotheses of the function of the contractile vacuole 

 have been developed. There is, first, some ground for the belief of 

 Spallanzani (1770), Bossbach (1874) and Dujardin (1841) that it is 

 an organoid having to do with respiration of the organism, together 

 with other possible functions, a view supported in modern times by 

 Biitschli (1877, L888) and Degen (1905). There is, second, ground 

 for the belief held by Stein (1859), Gruber (1889) and the majority 

 of modern students of Protozoa, that it is an organoid for the excre- 

 tion of katabolic waste, despite the unconvincing experimental evi- 

 dence by Brandt (1885), and by Griffith (1889). Howland (1924), 

 however, by using a much more delicate test (the Benedict blood- 

 filtrate test) obtained unmistakable evidence of the presence of uric- 

 acid in cultures of Protozoa; in P. caudatum analyzed by Benedict, 

 a color reaction was obtained equivalent to 4 to 5 mg. of uric acid 

 per liter. There was no proof here, however, that the uric acid 

 came from Paramecium. Weatherby (1929) showed that the usual 

 ingredients of a culture medium contain measurable quantities of 

 uric acid. He found, however, that the extracted fluids of con- 

 tractile vacuoles of Paramecium and Spirostomum contain urea, 

 whereas the vacuole of Didinium nasutum contains ammonia, but 

 in no case does the nitrogenous waste of the vacuole represent all 

 of the nitrogenous output of the cell, much being voided by exosmosis. 

 There is, third, ground for the belief that the contractile vacuole is 

 an organoid for the regulation of osmotic pressure in the cell, a view 

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