STUDY OF FERTILIZATION 433 



tozort to a high degree of activity : it is not, however, the sole 

 means whereby an increased activity may be brought about. 

 The same effect can frequently be obtained by exposing the 

 spermatozoa to a slight increase of hydroxyl ions in the sur- 

 rounding medium, in fact the activity of spermatozoa can 

 readily be regulated by this means (Gray, 9 ; Cohn, 3) ; the 

 gametes of Luidia have been frequently observed to show 

 no activity when exposed to egg-secretions, whereas intense 

 activity is aroused by hydroxyl ions (Gray, 11). Again, the 

 sperm of Sphaerechinus will not fertilize the eggs of 

 Strongylocentrotus, unless hydroxyl ions are added to 

 the medium. These facts indicate that either (i) the efficiency 

 of fertilizin depends upon the concentration of hydroxyl ions 

 present, or (ii) ' fertilizin ' is itself a weak base which stimulates 

 the spermatozoon by virtue of its basic properties. The second 

 hypothesis covers all the facts, has the advantage of simplicity, 

 and is supported by the fact that the essential constituent of 

 the egg-secretion is readily destroyed by acids, but not by 

 alkalies. 



Since, however, there is unanimous agreement that a certain 

 degree of activity on the part of the spermatozoon is necessary 

 for fertilization, let us consider the more unique effects of egg- 

 secretions on the sperm, viz. aggregation and agglutination. 

 There can be no doubt that the aggregation of the sperm into 

 active clusters must be due to some effect which the sperm have 

 on each other : the observations of Lillie on the spontaneous 

 aggregations of Nereis sperm provide strong evidence that 

 aggregation is due to the production of (JOg and that the sperm 

 aggregate at regions of optimum COg tension. There is nothing 

 to indicate that such clusters are due to any other cause than 

 the clusters of protozoa which have been described by 

 Jennings (12). It is, therefore, reasonable to suppose that 

 whenever the activity of the sperm is sufficient to produce the 

 critical amount of COg in the medium immediately surrounding 

 each sperm, then aggregations will form ; naturally, they will 

 only be temporary owing to (i) a gradual abatement in COg 

 production in an increasingly acid solution, (ii) a gradual 



