STUDIES OF FERTILIZATION 527 



always produces complete paralysis very quickly, and N/10000 

 always permits maximum activity. The range of activity with 

 reference to these acids is thus marked out fairly well. It will 

 be noted that acetic acid has a greater inhibiting effect than the 

 mineral acids. 



It is an interesting question in this connection whether there is 

 a certain optimum amount of acidity which increases rather than 

 decreases the activity of the spermatozoa. In the experiments 

 now under consideration this could not be determined certainly. 

 In some cases spermatozoa aggregated more rapidly in weakly 

 acid solutions (N/5000 and under) than in the control; in others 

 at the same rate or at a slightly less rate. In the experiments on 

 chemotaxis, however, which involve an acid gradient, there is 

 possible evidence of stimulation of weak solutions. 



CO 2. The sensitiveness of the spermatozoa to CO2 is con- 

 sidered separately because of its probable biological significance 

 and also because it was impracticable to state the strength of the 

 solution in molecular terms. The solutions were prepared empiri- 

 cally as follows : A certain quantity of sea-water was supersatura- 

 ted with CO2 in 'Sparklet' siphons. The charged sea-water was 

 drawn as desired, and after the effervescence ceased it was diluted 

 with measured quantities of sea-water, and the dilutions were 

 expressed as percentages of the charged sea-water. These solu- 

 tions were always prepared fresh for each experiment, and kept in 

 stoppered bottles or otherwise covered as far as possible. The 

 uniformity of the reactions obtained is adequate proof that the 

 solutions used in the different experiments were equivalent. 



A very large number of experiments was made with CO 2 during 

 the course of the summer, so that the relations of the spermatozoa 

 to CO 2 were more adequately ascertained than for any other 

 substance. Here the question is only of the relation of CO2 

 tension to the activity of the spermatozoa, and the results may be 

 stated as follows : 



One per cent of the charged CO 2 sea-water paralyzed the sper- 

 matozoa immediately; or rather a suspension of a drop of dry 

 sperm in 6 to 8 cc. of this strength of CO2 does not exhibit any 

 activity. This is, however, very near the minimum paralyzing 



