STUDIES OF FERTILIZATION 519 



II. ACTIVATION PHENOMENA 

 A. NEREIS 



1. The aggregation reaction 



We may begin the discussion of the behavior by describing 

 a phenomenon which was used throughout the experiments with 

 Nereis as a test of the activity of the spermatozoa. A drop of 

 dry sperm from a mature Nereis is mixed in about 6 cc. of sea- 

 water in a Syracuse watch crystal, making a uniformly milky 

 suspension ; in a few seconds clouds begin to appear and in fifteen 

 to forty-five seconds these usually draw together in dead-white 

 solid-looking masses uniformly spaced throughout the fluid. The 

 intervening fluid becomes quite clear and the masses quickly 

 settle on the bottom. The rate of formation of these masses, 

 their number and size, depend on temperature, 'freshness' of 

 the sperm and other conditions discussed beyond. Any sperm 

 suspension that exhibits the aggregation phenomenon will be 

 called 'aggregative' in the experiments that follow. 



The appearance is of course due to the aggregation of the sperm 

 in closely packed masses. Under a low power of the microscope 

 each mass appears like a swarm of bees, owing to the intense activ- 

 ity of the peripheral spermatozoa. But those in the interior of 

 the dense mass must be quiescent or the masses would break 

 apart. After the aggregations have settled to the bottom of the 

 crystal they tend to flatten out and may run together in time to a 

 greater or less extent. 



If, immediately after settling of the aggregations, the sperm 

 is mixed up with a pipette, a perfectly uniform milky suspension 

 is again produced, which may aggregate a second time, but more 

 slowly than the first time, and in fewer aggregates; and the inter- 

 vening fluid remains quite opalescent, showing that all the sper- 

 matozoa have not joined the aggregates. A third mixing up is 

 not usually followed by aggregation until after the spermatozoa 

 have settled to the bottom, and then only a veiy partial aggrega- 

 tion results. 



A considerable number of variations of this theme can be pro- 

 duced by using sperm suspensions of varying density contained 



