io6 



PROBLEMS OF FERTILIZATION 



It is clear from this experiment that the spermatozoa 

 react positively in a CO 2 gradient where the tension is 

 above a certain point; the aggregation caused by a 

 more concentrated drop grows because the diffusion of 



CO 2 from the center 

 furnishes a widening 

 ring of the necessary 

 concentration. To 

 furnish a gradient the 

 concentration of CO 2 

 must exceed that in 

 the sperm suspension, 

 which is a function 

 of its density and age ; 

 on the other hand, a 

 limit is set to the dif- 

 ferential which fur- 

 nishes the reaction by 

 the fact that a con- 

 centration of about 

 i per cent of the 

 C0 2 sea-water para- 

 lyzes the spermato- 

 zoa. The gradient 

 that determines the reaction must, therefore, exist within 

 very narrow limits. 



Such aggregation with reference to CO 2 is obviously 

 the same in principle as the aggregations formed spon- 

 taneously in sea-water. Indeed, aggregations after- 

 ward form spontaneously in the unaffected part of such 

 a preparation. The spermatozoa at the periphery of 

 the ring caused by CO 2 are in much more rapid move- 



FIG. 15. Diagram of the reaction of 

 spermatozoa of- Nereis to a drop of i per 

 cent CO 2 in sea-water. The dark back- 

 ground represents the sperm suspension; 

 the thick open circle and streak below, the 

 aggregation; the unbroken line represents 

 the original boundary of the CO 2 drop; the 

 concentric broken lines in the clear zone 

 represent the CO 2 gradient. 



