EGG-LAYING IN AMPHITRITE. 259 



means for scattering them by currents over the sand flats, and 

 their own locomotion aids them in their distribution. They 

 have sufficient motor power to aid in horizontal distribution, 

 and have sufficient density to prevent them from rising much 

 above the bottom where the eggs are found. That sperm have 

 such a method of distribution was substantiated in a different 

 way by studying the scattering of sperm in dishes of still water. 

 A large quantity of sperm was introduced at some point in the 

 bottom of the dish; then the progress in distribution was ob- 

 served by noting the advancing cloudiness of the water, reckoned 

 in various directions from the point of departure. It was found 

 that the advance in a general horizontal direction was many 

 times more rapid than the advance in an upward direction. I 

 take this to be due to the resistance of gravity. Since the sperm 

 are set free near the bottom under normal conditions, their 

 lightness tends to bring about more favorable chances for fertili- 

 zation of the eggs. 



We may now sum up the results of these specific gravity ex- 

 periments in the form of certain explanations and conclusions: 



1. The eggs of Amphitrite increase in specific gravity during 

 growth in the ccelome. This is probably due to an increase in 

 the amount of yolk. The reason therefore that the larger mature 

 eggs settle in a dish of sea-water more rapidly than the smaller, 

 immature ones, is undoubtedly due in part to the greater specific 

 gravity of the older eggs. And in the process of egg-laying it is 

 probable that a difference in specific density may act as a means 

 to separate ripe from unripe eggs. 



2. The reason why the larger eggs sink before the ccelomic 

 corpuscles must be explained in another way, for the specific 

 gravity of the corpuscles is decidedly greater than that of the 

 eggs. The difference in behavior between these bodies is to be 

 explained, I believe, by the flat, oblong shape of the corpuscles; 

 their shape is such that they offer in settling a much larger resist- 

 ance in proportion to their mass. Blood cells settle more slowly 

 than eggs in sea-water because of a difference in shape and not 

 because of a lesser density. Slight currents in the dish prevent 

 the corpuscles from settling for a long time, while they hardly 

 interfere with the downward movement of the eggs. At this 



