1910] HOYT—FERTILIZATION IN FERNS 355 
be able to obtain entrance of sperms into them; but no such case has 
been observed. 
It is well known that different degrees of fusion of egg and sperm 
are obtained in different combinations. In some cases apparent 
fusion occurs, but the offspring remain sterile; in others a partial 
fusion occurs, but the offspring fail to develop; while in others no 
fusion occurs. A good discussion of the cases where after fusion 
the offspring remain sterile or undeveloped is given by DEVRIES 
(7). Such cases seem to be due to some interrelation of the chromatin 
content of the two sexual cells. The failure of egg and sperm to 
fuse seems to be caused by some physical or chemical interaction 
between the two. It remains to be seen whether there is any sharp 
line between these two classes of phenomena. 
The possibility of fusion seems to be determined by different 
factors in different species of plants and animals. In some fish there 
is no specific adaptation; in echinoderms changes occurring after 
the removal of the eggs from the ovary permit the fusion of sperms of 
different species at different intervals of time; in some echinoderms 
the alkalinity of the sea water determines the possibility of the fusion 
of certain sperms; in some cases the relative surface tension of egg 
and sperm may determine the possibility of fusion; in other cases the 
presence of certain substances in the egg may prevent the fusion of 
certain sperms; and lastly double fertilization may be prevented by 
the formation of certain substances or by physical changes in the egg. 
The possibility of fusion seems, then, to be determined in some cases 
chiefly by physical, in others by chemical conditions. 
In ferns we have no evidence of the existence of any physical 
barrier in the ripe egg that might prevent the entrance of foreign 
sperms. In the present state of knowledge, it seems probable that 
the failure of egg and sperm to fuse is due to some interaction between 
the two. It is greatly to be hoped that we shall some day be able to 
obtain crosses. We may then be able to obtain some evidence con- 
cerning the nature of this interaction. 
C. Movements and reactions of sperms 
The accounts of the movements of motile organisms may in 
general be considered in two groups. In the first of these, the 
