180 BIOLOGY OF THE PROTOZOA 



and these form the chief basis of our knowledge of protozoan irri- 

 tability. 



Attempts to explain pseudopodia formation and ameboid move- 

 ment have varied with the changes in our conceptions of the physical 

 make up of protoplasm. The protoplasm of Ameba regarded as 

 a fluid substance was supposed to follow the laws of surface tension 

 characteristic of all fluids. Pseudopodia formation, according to 

 the views of Berthold (1886), is the attempt of one fluid (proto- 

 plasm) to spread out between water and the substratum as Quincke's ' 

 well-known experiments demonstrated for fluids. As physical con- 

 ditions on all sides of the Ameba are not equal, variations in tension 

 result in local diminution, and the tendency to spread is focussed in a 

 local area and the pseudopodium results. Biitschli's (1894) observa- 

 tions and experiments with emulsions of oil, salts and water, and 

 Rhumbler's (1898) analysis of the causes of movement in lobose 

 rhizopods led these observers also to interpret pseudopodia forma- 

 tion as a result of surface tension phenomena. With the more 

 modern conception of protoplasm as a colloidal aggregate in the 

 physical state of an emulsoid in which the external and internal 

 protoplasm of Ameba are in the relation of gel and sol, the difficulty 

 of applying the laws of fluids became apparent and the hypothesis 

 based upon surface tension has been generally abandoned. Rhum- 

 bler himself (1910 and 1914) recognized this difficulty and materi- 

 ally changed his conception of ameboid movement, while Hyman 

 (1917) greatly enlarged and perfected his later point of view. 

 According to Hyman the ectoplasm of Ameba, by virtue of its 

 relatively solid state, becomes tenuous but elastic, as demonstrated 

 by the experiments and observations of Jennings (1904), Kite (1913), 

 Schultz (1915) and Chambers (1915, 1917), and exerts an elastic 

 tension on the inner fluid protoplasm. Bancroft (1913) and Clowes 

 (1916) demonstrated the reversibility of phase in diphasic physical 

 systems through the agency of electrolytes, and the conclusion fol- 

 lowed that \he ectoplasm represents a reversal phase of the more 

 fluid inner protoplasm. Hyman argues that, owing to the tension 

 of the enveloping ectoplasm, if any local region of the more solid 

 ectoplasm becomes liquefied, the resistance gives way at such a 

 point and the fluid endoplasm is pressed out, thus forming a pseudo- 

 podium. The immediate cause of such liquefaction she traces to 

 a local increase of, or change in, metabolic activity resulting in the 

 production of hydrogen-ions which, with the surrounding medium, 

 form an acid appropriate for dissolution of the more solid ectoplasm. 

 By the use of Child's potassium cyanide test for metabolic gradients, 

 she was able to demonstrate that such local regions of greater meta- 

 bolic activity actually occur on the periphery of Amoeba proteus 

 before a pseudopodium breaks out, also that the extreme tip of the 

 advancing pseudopodium is the most actively metabolic part. 



