1910] HOYT—FERTILIZATION IN FERNS 353 
are practically incapable of fertilizing the eggs of the same individual 
and of certain other individuals, and has suggested that this is due 
to substances present in the egg furnishing a sort of “immunity” 
against these sperms. 
NEWMAN (35) was able, by treating the eggs of Fundulus with 
solutions of salts that precipitate colloids and increase the surface 
tension, to destroy the permeability of the egg membrane to the 
sperm, and then, by treating such eggs with solutions of salts that 
dissolve precipitated colloids and decrease surface tension, to restore — 
the permeability of the membrane. The results of MoRGAN on Ciona, 
according to this author, are capable of being explained by the 
assumption that different eggs and sperms have different relative 
surface tensions. NEWMAN states that “these experiments and those 
of LorB and MorGan seem to indicate that the relative surface 
tension of the egg and of the sperm is one important factor governing 
fertilization.” 
Moenxuaus, however, in cross-fertilizing fishes that in many 
cases belong to widely separated orders, often found the percentage 
of eggs impregnated to be as great as in the check experiments where 
the sperm and eggs were of the same species. When eggs of Fundulus 
heteroclitus were subjected to a mixture of equal parts of: sperms of 
the same species and of sperms of Menidia notata or M. gracilis, 
there were often more fertilizations by the foreign sperms than by 
the sperms of Fundulus. The foreign sperms were prepotent. 
Moenxkuaus concluded that, among the forms used in the experi- 
ments, there is no specific adaptation such as chemical affinity, 
surface tension, etc., but that the prepotency of the foreign sperm 
is due to its being more active and hence a swifter fertilizer. 
TENNENT (49) was unable to obtain crosses between different 
Senera of echinoderms when the sperms were added to the eggs 
immediately after their removal from the ovary, but obtained fusion 
in every combination tried when the eggs had been allowed to stand for 
Several hours. The time after removal from the ovary when fertiliza- 
tion with foreign sperms can be obtained is definite for any particular 
combination and is different in different combinations; it appears 
to depend on the species of sperm as well as on the egg employed. 
The eggs, after standing and becoming capable of fertilization by 
