178 BIOLOGY OF THE PROTOZOA 



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

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



These three beliefs are not necessarily exclusive and the possibil- 

 ity of all three functions is still open. The osmotic function is well 

 supported by evidence furnished by Gruber's (1889) experiments 

 in transferring fresh-water, vacuole-holding Actinophrys sol and 

 Amoeba crystalligera to salt water, and vice versa, or by Zuelzer's 

 similar experiment with Amoeba verrucosa, the protoplasm becom- 

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

 (1923) found that Vahlkampfia calkensi 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 w T ith varied conditions of the environment 

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

 pressure, and irrespective 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, a conclusion strengthened by Botsford's (1926) mer- 

 otomy experiments with Amoeba proteus in which it was shown that 

 the size of the vacuole depends upon the size of the fragment cut 

 off. According to Degen and Hartog it is the function of the 

 contractile vacuole to establish this balance. 



This hypothesis, with further evidence supplied by 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 

 contains water-soluble, katabolic excretory substances such as C0 2 

 and nitrogenous waste, positive evidence for which is supplied by 

 several observers. This indeed was admitted by Degen although he 

 obtained no evidence of the nature of the substances excreted. He 

 saw in the membrane of the vacuole the possibility of an excretory 

 mechanism. The actual existence of such a membrane, however, 

 is still in dispute, indeed the majority of investigators deny its 

 existence (Biitschli, Khumbler, Schewiakoff, Taylor). Others, 

 however, give evidence to show that a true membrane, although 

 very delicate, is actually present. Howland (1924, 1) for example, 

 by micro-dissection methods has been able to remove the contractile 

 vacuoles of Amoeba verrucosa and of Paramecium caudatum after 

 which they retain their integrity for considerable periods as free 

 vacuoles in the surrounding water. She also has punctured the 

 vacuole with needles while in the endoplasm, causing the expulsion 

 of its contents into the surrounding endoplasm and resulting in the 

 wrinkling of the vacuole membrane. Nassonov (1924) also not 



