STUDIES OF FERTILIZATION 543 



6. Thermotaxis 



The spermatozoa of Nereis do not exhibit any positive response 

 to drops of sea-water at higher temperatures. Into suspensions 

 of spermatozoa in sea-water at 21°C. under raised cover-shps 

 drops of sea-water at 44°C., 52°C., and 84°C. were injected succes- 

 sively.. No ring formation occurred with reference to any of these 

 drops. They did not fill up with sperm as rapidly as drops of 

 sea-water at room temperature, but this is no doubt due to the 

 paralysis that sets in, as previously noted above about 28°C. 

 The only observable effect of the heated drops was that aggrega- 

 tions formed a little ear her in the general suspension near the 

 margins of the introduced drop; and this is attributable to in- 

 creased activity of the sperm owing to rise of temperature, hence 

 increased CO2 production in the zone of increased activity; and 

 more rapid aggregation as a consequence. 



7. Thigmotaxis 



In contact with any solid object Nereis spermatozoa tend to 

 carry out circus movements in an anti-clockwise direction, when, 

 fresh, but may soon come to rest. In any field of the microscope 

 in a suspension beneath a cover glass one sees many of the sper- 

 matozoa in contact with the slide at rest, and many others carry- 

 ing out the circus movements, while those that are freely sus- 

 pended swim in spiral paths. The thigmotatic reaction then 

 appears first to be the exaggeration of the i:otation component of 

 the ordinary locomotor movements, and second rest. 



This reaction may of course come in conflict with the chemo- 

 tactic reaction, as for instance in the clear margin external to the 

 ring of spermatozoa produced in response to a drop of 1/100 CO2 

 sea-water. Within this area all the freely suspended spermatozoa 

 swim directly towards the ring, but those in contact with slide or 

 cover-slip may continue their circus movements without any 

 apparent directive effect from the CO2 gradient. The thigmo- 

 tactic stimulus appears thus to be more effective than the CO2 

 gradient. 



