156 
ZOOLOGY: F. R. LILLIE 
THE FERTILIZING POWER OF SPERM DILUTIONS OF ARBACIA 
By Frank R. Lillie 
DEPARTMENT OF ZOOLOGY, UNIVERSITY OF CHICAGO 
Read before the Academy, December 9, 1914. Received January 15, 1915. 
My previous studies of fertilization led to the conclusion that initia- 
tion of development depends upon activation of a substance produced 
by the egg and located in its cortex. This substance, for which I pro- 
pounded the name fertilizin, possesses two side-chains involved in fer- 
tilization, one of which unites with receptors borne by the spermatozoon, 
the other with the receptors borne by the egg. The latter is the fertili- 
zation reaction proper; it was postulated that any agent that may acti- 
vate the fertilizin, as do the sperm receptors, so as to cause its ovophile 
side-chain to combine with the egg-receptors may act as a partheno- 
genetic agent. Fertilization and parthenogenesis are thus brought under 
one point of view with reference to initiation of development. 
Certain data concerning the fertilizin of Arbacia, such as time of ori- 
gin, its location in the egg, its disappearance after fertilization, etc., are 
known owing to its property of agglutinating spermatozoa of the same 
species, which serves as indicator. Some of its chemical and physical 
properties have also been studied to a certain extent. But up to the 
present the very existence of the so-called sperm receptors has remained 
hypothetical. On any theory of fertiHzation it is necessary to postulate 
the existence of a spermatic substance that induces development of the 
egg. But in spite of a considerable number of investigations, the exist- 
ence of such a substance still remains an hypothesis. The present con- 
tribution presents demonstrative evidence, though of a negative sort, 
concerning this substance. 
The phenomena, on which I rely for my conclusion, concern the fer- 
tilizing power of sperm suspensions of graded dilutions. On the basis 
of the usual supposition that a single active spermatozoon may fertiUze 
an egg of its own species, all of the eggs should be fertilized in a series of 
sperm suspensions of increasing dilutions up to the place in the series 
in which each egg receives only a single spermatozoon. Beyond this the 
percentage of eggs fertihzed should fall off at a certain rate to a vanishing 
point. 
An approximate reahzation of this may be obtained if the interval 
between preparation of the more dilute sperm suspensions and their use 
in fertilization be made as short as possible. This may be done by the 
addition of eggs to measured quantities of sea-water, followed by the 
