STUDIES OF FERTILIZATION 545 



ally of some rheotactic reaction. But on more careful examina- 

 tion and consideration the following explanation appears much 

 more probable. Microscopic aggregations must begin to form 

 while the water currents are still moving; they are then elongated 

 by the friction in the direction of the current, and as they grow 

 to macroscopic size the aggregations tend to preserve this form. 

 The definition of the currents is due to the form of the aggrega- 

 tions rather than to their arrangement, and as they contract to 

 spherical form the current-figures become less pronounced and 

 very largely disappear. 



Very interesting configurations may be produced in a sperm 

 suspension of Nereis by dropping in dilute acids. In a few seconds 

 quite complex wreath-like or festooned aggregations of sperma- 

 tozoa appear at the site of the entering drop marking out accur- 

 ately the disti'ibution of the acid in the suspension. These of 

 course vary with the strength of the acid, and the distance from 

 which it is dropped. 



If a few drops of a suspension of active Nereis sperm be mounted 

 beneath a raised cover slip, it will be observed that the outer 

 inai'gin of the suspc^ision for a width of 1 to 2 mm. soon becomes 

 free from spermatozoa, thus tending to concenti'ate the suspen- 

 sion the same distance from the margin (fig. 4). This concen- 

 trated ring of the suspension then tends to form aggregations more 

 rapidly than the more central parts. In the case of a suspension 

 that is not perfectly fresh, aggregations may form only in this 

 ring. The withdrawal of spermatozoa from the margin of the 

 drop might at first thought be attributed to a negative chemotaxis 

 towards oxygen. Howe\'er, it is almost cei'tainly not this, l)ut 

 a positive reaction towards the higher COj tension of the interior 

 of the drop. If a drop of sea-water saturated with oxygen be 

 injected into a suspension beneath a raised cover, the spermatozoa 

 avoid it in the same way that th(\\' do tlu^ free margin of the 

 suspension. 



The spermatozoa of Nereis make an acid indicator more deli- 

 cate than any of the chemical dye indicators. In the course of 

 some experiments I discovered quite accidentalh', thus avoiding 

 an awkward mistake, that the first few drops of water through any 



