182 Experimental Zoology 
by Loeb. If to 100 parts of an artificial sea water containing 
0.3 cubic centimeter sodium hydroxide the eggs of Strongylo- 
centrotus and the sperm of the starfish be added, as many as 50 
to 80 per cent of the eggs will be fertilized. The eggs segment 
and it appears that even the early stage of development — more 
or less abnormal — may be reached. 
Polys permy 
In the great majority of known casesonly a singlespermatozoén 
normally enters the egg to fertilize it. The moment one sperma- 
tozo6n penetrates, some reaction takes place in the egg that pre- 
vents other spermatozoa from entering. The reaction involves, 
in some cases, contraction of the egg from the surrounding mem- 
brane; in other cases the formation of amembrane; and we may 
infer that other kinds of changes in the protoplasm may in other 
cases interfere or prevent the entrance of more than one sperma- 
tozodn. If the egg is injured or narcotized or is stale, more than 
one spermatozoén may enter. The result is generally injurious, 
for, while a regular or more often an irregular cleavage may fol- 
low, the embryo fails to develop properly. This failure seems 
to be due, in the main, to the unequal distribution of the chroma- 
tin or other materials to the different parts of the egg. 
It has been found in hybridizing that in certain cases more 
than a single sperm enters the egg, and the irregularities in 
cleavage that follow seem to be due in part to this condition, but 
in other cases where one spermatozoén enters, the failure to 
develop must be ascribed to different causes. In these cases 
the results may be due sometimes to irregularities in the mechan- 
ism of division, but at other times to some incompatibility or 
failure of the two uniting elements to work together to a com- 
mon end. 
According to the degree of perfection to which the distribution 
of the materials of the egg is carried out, the subsequent develop- 
ment is more or less perfect. Even in cases where the hybrid 
reaches the adult condition, it has been observed that it may have 
a weak constitution, and even when it is strong the hybrid is 
