254 JOHN W. SCOTT. 



However a theoretical explanation was given in which the 

 nephridia were regarded as "settling basins," the cilia in this 

 case preventing the settling of bodies which were not to be ex- 

 pelled. This explanation was based partly upon the structure 

 of the nephridia, and partly upon the fact that "when the con- 

 tents of the ccelome are stirred in sea-water, the largest ova 

 including the ripe eggs, always settle more quickly to the bottom 

 of the dish ; the immature eggs then settle and last of all the 

 ccelomic corpuscles." The important fact here is that gravity 

 more quickly influences the large ova than the other bodies 

 floating in the ccelomic fluid. It was suggested that the difference 

 in the rate of settling was probably due to a greater density of 

 the ripe eggs, but that the effect was possibly due to a difference 

 in the shape of the bodies concerned. In either case, the con- 

 clusion was drawn that gravity is a differential means of separa- 

 tion. However, this left the matter inconclusive and the question 

 arose, Is this tendency of large ova to settle quickly due to a greater 

 specific gravity, or to a greater mass in proportion to the amount of 

 surface offering resistance? To answer this was the primary object 

 in the next step of the investigation. 



SPECIFIC GRAVITY EXPERIMENTS. 



Having set the problem, I began the work of determining the 

 specific gravity of the eggs and corpuscles found in the fluid of 

 the body cavity. Incidentally the specific gravities of sperm, 

 and of eggs in some later stages of development were obtained. 



Lyon's plan of getting the density of eggs by centrifuging in 

 gum arabic solution was tried and, after finding it suited my 

 purpose, his method was adopted with slight modifications. 

 First a strong gum arabic solution in sea-water was prepared and 

 its density carefully determined. From this stock solution a 

 series of stock solutions was made up of differing densities; this 

 can be readily done by mixing with the stock solution in proper 

 proportions sea-water of known density. When ready to cen- 

 trifuge the capillary tubes of a haematocrit were filled about 

 three fourths full of gum arabic solution. On top of these 

 solutions whose density was known was placed the material 

 whose density was required. In a few cases, in order to guard 



