12 



afternoon to a much greater extent than early in the day; 

 before two o'clock in the afternoon the hovir in the dark 

 would sometimes bring about the deposition of a very small 

 number of eggs, and if the period of darkening was length- 

 ened to an hour and a half, a slightly larger number of 

 eggs were foiind in the water. But after two o'clock an 

 hour's shutting away from the light brought about an appa- 

 rently normal spawning. I fovmd that the stimulus of the 

 withdrawal of light is surprisingly definite in its ef- 

 fects; the condition of the tissues arrives at the point 

 requisite for the release of the eggs almost on the minute. 

 This constancy is not appreciably affected by ra<Jderate 

 changes in temperature. A large munber of experiments and 

 observations have been made to educe the exact time of 

 stimulation (if we may so speak of an influence which seems 

 to be purely negative) and the results are summarized in 

 the following table. Record was kept of experiments car- 

 i-ied on during the whole of the fertile season, partly in 

 one summer, partly in the next. The stimulation-time va- 

 I'ies somewliat with the season; the table gives the results 

 obtained during the last week in July, when the eggs were 

 being discharged in the greatest numbers. 



