102 E. E, JUST. 



This observation led to a series of experiments (during 1913 and 

 1 9 14) designed to ascertain whether or not the density of the 

 sperm suspension is a factor in the fertihzation of Platynereis. 



These experiments prove in general that the number of dry 

 eggs added to sperm suspensions that develop depends upon the 

 density of the suspension. The denser the suspension the larger 

 the number of trochophores. Moreover, for dense suspensions 

 the minimum amount of sea-water permitting fertilization appears 

 to be slightly higher than for thin suspensions. Cleavage is 

 directly a function of the chances of the spermatozoa reaching 

 the egg before the fertilizing substance is lost. 



The time of flooding with sea-water after insemination Is also 

 important for the highest percentage of cleavage. But these 

 factors cannot be expressed with mathematical exactness. Some 

 points, particularly with reference to inseminations with dense 

 suspension need further experiments to determine their signi- 

 ficance. 



That the egg when exposed to the action of sea-water quickly 

 loses something necessary for fertilization must be the conclusion 

 drawn from these experiments with washed or unwashed eggs. 

 Even thirty seconds residence in sea-water, as repeatedly proved, 

 is sufficient to inhibit cleavage in every single egg. If dry eggs 

 from a single female be put in five cubic centimeters of sea-water 

 and thoroughly drained as soon as they settle they will not de- 

 velop after insemination although this procedure may take but a 

 half minute. The egg alone is affected by sea-water; the fer- 

 tilizing power of the sperm Is not affected by exposure to sea- 

 water. 



3. The Nature of the Fertilizing Substance. 



The fertilizing substance once lost cannot be restored. If 

 washed eggs be mixed with an extract obtained by crushing dry 

 eggs in one or two drops of sea-water and dry sperm added, 

 cleavage does not result. I lay no stress on this, however, for 

 It seems to me that such an extract might yield anything. 



The presence of various substances in the sea-water or the 

 lowering of the temperature of the sea-water does not prevent or 

 restore the loss of this substance. 



