170 BIOLOGY OF THE PROTOZOA 



incongruous than the supposition of Ehrenl^erg (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 ^'itality, more 

 reasonable hypotheses of the function of the contractile vacuole have 

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

 Spallanzani (1776), Rossbach (1874) and Dujardin (1841) that it is 

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

 with other j)ossible functions, a view supported in modern times by 

 Biitschli (1877, 188S) 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 

 excretion of katabolic waste, despite the unconvincing experimental 

 evidence by Brandt (1885), and by Griffith (1889). Rowland (1924), 

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

 filtrate test) obtained unmistakable evidence of the presence of uric 

 acid in Protozoa; in P. caudatum analyzed by Benedict, a color reac- 

 tion was obtained equivalent to 4 to 5 mg. of uric acid per liter. 

 There is, third, groimd for the belief that the contractile vacuole is an 

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

 advanced by Hartog (1888) and supported by Degen (1905), Stempell 

 (1914), Khainsky (1910) and recently by Nassonov (1924). 



These three beliefs are not necessarily exclusive and the possi- 

 l)ility of all three functions is still open. The osmotic function is 

 well supported by evidence furnished by Gruber 's (1889) experi- 

 ments in transferring fresh-water vacuole-holding Adinophrys sol 

 and Amoeha crystaUigera, to salt water, and tice versa, or by Zuelzer's 

 similar experiment with Amoeha verrucosa, the protoplasm becom- 

 ing more condensed and the vacuole lost in salt water. Hogue 

 (1923) foimd that Vahlkamyfia calhensi when transferred from salt 

 water to fresh-water media developed 1, 2, 3, or even 4 contractile 

 vacuoles. More extensive experiments by Degen (1905) with salts 

 of different kinds and with varied conditions of the environment, 

 show that the contraction of the vacuole is a function of osmotic 

 pressure, and irrespecti^^e of the type of salt or neutral solution 

 introduced. With Hartog, he concludes that protoplasm of fresh- 

 water forms, with its salts in solution, has a higher osmotic pressure 

 than the surrounding medium, which leads to continued intake of 

 water. Such intake, if not balanced, would lead to inflation and 

 to diffluence. According to Degen and Hartog it is the function 

 of the contractile vacuole to establish this balance. 



This hypothesis, with further evidence supplied b>' the absence 

 of contractile vacuoles in marine forms where osmotic relations of 

 protoplasm and environment are more evenly balanced, is theoreti- 

 cally correct. There is no reason to doubt, however, the further 

 possibility that the water expelled by the contraction of the vacuole 



