176 EDWIN J. COHN. 



crease in the hydrogen ion or carbon dioxide concentration or 

 increase in the oxygen concentration increases the activity of 

 spermatozoa. Three explanations of the configuration that 

 appears when spermatozoa are subjected to such a gradient in 

 carbon dioxide as has been described have been suggested; (a) 

 that spermatozoa are activated in certain concentrations of car- 

 bon dioxide; (&) that spermatozoa are positively chemotactic or 

 chemotropic to carbon dioxide; (c) that the accumulation of 

 spermatozoa at a certain concentration of carbon dioxide is 

 brought about by their inactivity in that concentration of carbon 

 dioxide. 



While it is not inconceivable that spermatozoa are activated 

 in certain concentrations of carbon dioxide, no evidence of such 

 a primary stimulation of spermatozoa has ever been observed 

 or reported. 



The circumstances which led to the formulation of the second 

 hypothesis are quite intricate and will require an historical in- 

 troduction if the problem is to be understood. Ever since 

 Pfeffer "demonstrated the importance of the part played by 

 chemotactic stimuli in causing the spermatozoa of liverworts, 

 mosses, ferns, etc., to approach the oospheres" (quoted from 

 Duller, A. H., 1902, p. 145) biologists have tacitly assumed or 

 attempted to demonstrate that this chemotactic phenomenon is 

 general in fertilization not only in plants 1 but in animals. In 

 1895 Bergh suggested that "the spermatozoa collect around the 

 ripe eggs, probably attracted by a special substance "(quoted 

 from Duller, A. H., 1902, p. 146). Three years later Massart 

 (Massart, J., 1888) demonstrated that the spermatozoa of the 

 frog were positively thigmotactic to glass. He was, however, 

 unable to demonstrate chemotaxis. This observation had pre- 

 viously been made by Dewitz (Dewitz, J., 1886). Massart also 

 maintained that spermatozoa were positively thigmotactic to agar 

 and gell (Massart, J., 1888) especially that of the egg (Massart, 

 J-, 1889). 



1 There is some doubt that chemotaxis is a general phenomenon in the fertiliza- 

 tion of plants. To a recent study of the "Physiology of Fucus Spermatozoids " 

 the following summary is appended. "Using the Pfeffer capillary tube method of 

 determining chemotaxy, it \vas found that certain acids cause collection of Fucus 

 spermatozoids. It is suggested that this may be explained as due to toxicity and 

 not chemotaxy" (Robbins, W. J., 1916, p. 130). 



