IMMUNITY AND SUSCEPTIBILITY OF SEA-URCHIN EGGS. 2 29 



pressure. It is believed that on this basis the varying effect of 

 ether on the eggs of Arbacia at successive periods after fertiliza- 

 tion can be explained. 



EXPERIMENTAL. 



i. The Effect of Ethyl Ether. 



The general methods of experimentation may be outlined as 

 follows : First, a slightly supersaturated solution of ether in sea 

 water was prepared, kept tightly corked, and when used a por- 

 tion was drawn from the bottom, thus ensuring a saturated solu- 

 tion. The eggs were fertilized in the usual way, good lots from 

 among a number being selected. At successive periods these 

 eggs were transferred to staining jars containing 50 c.c. of the 

 solution used. The strength of the solution actually used was 

 accurately controlled by starting with twice that strength and 

 then diluting exactly one half, in part with the sea water neces- 

 sary for transferring purposes. The eggs were allowed to stand 

 in these covered jars for the length of time selected ; the solution 

 was then carefully drawn off; the eggs were thoroughly washed 

 twice with sea water, which was again added, and given time to 

 develop. In important experiments the lots were each observed 

 twice. All the experiments were conducted at the room tem- 

 perature, about 20 C. 



The practical problem presented was to get such a strength of 

 solution and time of exposure that of the eggs transferred at the 

 various periods after fertilization some would be stopped in their 

 development, others not. For each value of the one factor there 

 would probably be a corresponding value of the other, the prod- 

 uct being a constant. An abstract record of what were essenti- 

 ally preliminary experiments is given on the next page. 



From these results it seemed that the correct relation between 

 the two variables, time and solution strength, had been found ; 

 accordingly in the next experiment a one sixty-fourth saturated 

 ether solution was used for 25', which gave very satisfactory 

 results. These are tabulated and plotted on page 008. 



In the plotting of the curve of these results the abscissae repre- 

 sent the times of transferral after fertilization, the ordinates the 

 per cent, of swimmers found by as accurate observation as possible. 



