354 BOTANICAL GAZETTE [May 
foreign sperms, are still able to be fertilized by their own sperms. 
Apparently changes take place progressively in the egg after its removal 
from the ovary, so that at different times it is capable of being 
fertilized by different species of foreign sperms, but these changes 
do not prevent the entrance of its own sperms. In a paper soon to 
be published, TENNENT considers the effect of variation in the con- 
centration of OH ions in the sea water in which the eggs stand before 
and during fertilization, and suggests that the variation in alkalinity 
brought about by artificial means in the experiments may imitate a 
natural seasonal variation in the concentration of the OH ions in 
ordinary sea water. 
In crossing the eggs of the starfish Asterias ochracea with the . 
sperms of the sea urchins Stronglyocentrotus purpuratus and S. 
jranciscanus, LOEB (26) found that in sea water the sperms would 
fuse with the eggs of their own species, but not with those of the 
starfish; while in sea water made slightly alkaline, the sperms would 
unite with the eggs of the starfish, but not with those of their own 
species. 
The failure of fern sperms to fuse with eggs of their own species 
which are just past maturity seems to be due, in part, to the physical 
condition of these eggs. Eggs which are evidently good have clear, 
soft-looking surfaces; while eggs as they become bad have darker 
surfaces that resemble more and more the appearance of a dri 
colloid. While it may be safely assumed that chemical changes are 
taking place within the egg at the same time, these physical changes 
would alone probably be sufficient to prevent the entrance of 
sperms. 
We have no evidence to help us to decide whether a similar explana- 
tion would account for the failure of a second sperm to enter an €88 
of its own species, since no constant difference between fertilized 
and unfertilized eggs was detected. An attempt to apply the same 
explanation to the lack of fusion of foreign sperms meets with still 
greater difficulties. A careful scrutiny failed to show that ¢88° 
of other species, which as mentioned were not entered by sper™s, 
had thicker membranes than did the eggs of their own species. it 
such a structure were present, we should find eggs of other speci® 
with thinner membranes than those of their own species, and should 
