1911.] Gametogenesis of the Gall-Fly, N. lenticularis. 487 
male homozygous. This scheme is perfectly satisfactory as long as no case is 
known in which there is evidence from breeding experiments that both the 
male and female are heterozygous. Hagedoorn* has described such a case in 
fowls, but his account is not very clear and no details are given. It indicates 
that a heterozygous iemale, paired with a homozygous recessive male, trans- 
mitted the dominant character only to its male offspring; the heterozygous 
males so produced transmitted, in the next generation, the dominant character 
only to their female children. This, if correct, can only be explained on the 
assumption that both sexes are heterozygous for the sex-determinants, and 
that the dominant character is always associated in this instance with the 
determinant which I have represented by the symbol ¢. The hypothesis of 
the American writers would then fall to the ground. 
Apart from this case, there is no certain means of deciding between the 
two hypotheses ; that of Wilson, Castle, and Morgan is simpler in not requiring 
selective fertilisation, for which little evidence exists; but until many 
more cases of sex-limited inheritance are known, and it is found that they 
never indicate that the heterozygous condition can exist in both sexes of the 
same species, both hypotheses accord equally well with the known facts. 
One further possibility should be mentioned, which does not differ greatly 
from that suggested by Morgan in the paper referred to above, and 
also more recently by Montgomery.t This is that the “factors” which 
have been referred to as sex-determiners are not really such, but that every 
individual contains potentially the character of both sexes—is in fact 
potentially hermaphrodite—but for the appearance of one or other of these 
characters additional factors are required. If the factor which I have called 
the ? determinant is present, it brings out the female characters and 
suppresses the male, if the ¢ determinant is present in the absence of 
the ?, it brings out the male characters. On such a hypothesis as this the 
results arrived at by Geoffrey Smitht would be explained. He infers 
a “sexual formative substance” in Malacostracan Crustacea, which causes 
the appearance of female characters; when a male is parasitised by 
Sacculina the parasite induces the secretion of such a substance by the 
organism, and the animal assumes the female characters and may even 
produce eggs in its testis. Even though such a male might, on the hypothesis 
summarised above, have the constitution ¢ ©, 7. might not contain the 
factor which brings out the female characters, yet the secretion induced 
by the parasite might have the same effect as this factor, and thus‘cause the 
* ¢ Archiv f. Entwicklungsmechanik,’ 1909, vol. 28, pp. 18, 26. 
t+ ‘Biological Bulletin,’ 1910, vol. 19, p. 9. 
{ “Studies in the Experimental Analysis of Sex,” ‘Q.J.M.S.,’ 1910, vol. 55, p. 225. 
