SEX-DETERMINATION IN AMPHIBIANS ooo 
cleavage plane, and such an absorption of water was not possible 
under the conditions to which these eggs were subjected. Unless 
by chance, therefore, in picking out the individuals to be reared, 
I selected in each case tadpoles that would give a great majority 
of females when developed, I can see no alternative but to as- 
sume that sex in Bufo can be altered by changing the water con- 
tent of the eggs at the time of fertilization. The weight of recent 
experimental and cytological evidence is, however, decidedly 
against the view that external factors can have any influence what- 
ever in determining sex. 
In making these experiments the spermatic fluid was distributed 
over the eggs within two or three minutes after they had been 
taken from the female. It is probable, therefore, that each egg 
was fertilized by the first spermatozoan that reached it, since in 
such a short space of time the well protected eggs could not lose 
sufficient water from evaporation to make selective fertilization 
possible, unless it be that fertilization is normally selective when 
the eggs of Bufo are fertilized. If the male is responsible for sex, 
each batch of eggs might have been expected to give a nearly 
equal proportion of the sexes, regardless of the external conditions 
to which they were subjected at the time of their fertilization; 
for former experiments have shown that, if the spermatozoa of 
Bufo are dimorphic, both kinds of spermatozoa must be produced 
in approximately equal numbers in each testicle of every normal 
male (King, 711). In each case, however, as indicated in table 
5, the individuals carried through to metamorphosis contained 
a proportion of females greatly in excess of that in any control lot 
as yet examined and much beyond the limits of probable normal 
variation. The chromosome theory of sex-determination does 
not, therefore, offer a satisfactory explanation of these results, 
unless one arbitrarily assumes that the lot of spermatozoa used 
in fertilizing each lot of eggs happened to contain a much greater 
number of female-producing spermatozoa than of those that were 
male-producing. 
The results of the experiments in which eggs were fertilized out 
of water, taken in connection with those obtained when eggs were 
subjected to the action of hypertonic solutions before fertilization, 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 12, No. 3 
