IMPROVED MeEtTuHop oF ARTIFICIAL PARTHENOGENESIS 71 
It is, moreover, necessary that not too many eggs be put 
into one bowl of hypertonic sea-water, since otherwise a mutual 
struggle for oxygen takes place. The eggs should also be kept 
in shallow dishes so that the layer of water which covers them 
may not be too deep and the diffusion of oxygen from the air to 
the eggs not too greatly hindered. I usually cover the bowls 
loosely with a glass plate." 
I have since tried the same method with success on the eggs 
of Arbacia at Woods Hole. As was to be expected, the quanti- 
tative data of the method are a little different in this case from 
those found in S. purpuratus. The eggs were put into a mix- 
ture of 50 c.c. sea-water+2 ¢.c. N/10 butyric acid for from one 
and one-half to three minutes. When transferred to sea-water 
they did not form a conspicuous fertilization membrane, as did 
the eggs of S. purpuratus under the same circumstances, but 
only a fine gelatinous layer which was not easily visible. Ten 
or fifteen minutes later the eggs were transferred to hypertonic 
sea-water (50 c,c. sea-water+8 c.c. 25 m NaCl). In this solu- 
tion the eggs remained at a temperature of about 23° only from 
seventeen and one-half to twenty-five minutes, after which 
they were transferred to normal sea-water. Since the time 
during which the eggs must remain in the hypertonic solution 
varies for the individual eggs and since a slight over-exposure 
in the hypertonic solution seriously injures the eggs, it is advis- 
able to transfer the eggs not all at the same time but at intervals, 
e.g., after seventeen and one-half, twenty, twenty-two and 
one-half, and twenty-five minutes. Eggs which were exposed 
longer than twenty-five minutes to the hypertonic solution did 
not develop. 
The same method has been applied successfully, among 
others at Naples by Herbst, Warburg, and by Meyerhof, and 
at Plymouth by Shearer and his pupils. 
1 Other factors which have to be considered in this method will be mentioned 
in the next chapter. 
