894 
Journal of Agricultural Research 
Vol. XXX, No. 10 
REVIEW OF LITERATURE 
THEORIES RELATING TO THE DETERMI¬ 
NATION OF SEX 
The phenomena of sex play such a 
large part in human life that they have 
attracted the attention of investigators 
and philosophers since the earliest 
time, and it is not surprising to learn 
that there have been many theories 
proposed to account for the determina¬ 
tion of sex. Geddes and Thomson (II) 3 
estimate that there were as many as 
500 of these theories at the beginning 
of the nineteenth century and state 
that the number has continued to 
increase. Undoubtedly the increase 
had not stopped when they made 
that statement, for the theory which 
is most widely accepted by scientists 
at present, the chromosome theory of 
sex determination, was not suggested 
by McClung {28) until 1902, a year 
after the revised version of Geddes and 
Thomson’s book was published. The 
only one of all these theories that has 
succeeded in winning any considerable 
amount of both popular belief and ex¬ 
perimental support is the chromosome 
theory of sex determination. The ex¬ 
perimental work upon which this paper 
is based rests upon the chromosome 
theory as a working hypothesis. 
According to that theory the sex of 
the individual which results from the 
union of the egg and spermatozoon is 
determined by whether it contains two 
X-chromosomes or, an X-chromosome 
and a Y-chromosome. Thus, in the 
mammals, females contain two 
X-chromosomes in each of their cells, 
and each of the egg cells which they 
produce contains one X-chromosome. 
Males contain an X-chromosome and a 
Y-chromosome in each of their cells, 
and half of the spermatozoa which 
they produce contain an X-chromo- 
some and the other half of the sper¬ 
matozoa contain a Y-chromosome. If 
the egg cell is fertilized by an X-bearing 
spermatozoon, the resulting individual 
will contain two X-chromosomes in 
its cells and will be a female. If the/ 
egg cell is fertilized by a Y-bearing 
spermatozoon the resulting individual 
will contain an X-chromosome and a 
Y-chromosome in its cells and will be 
a male. Sex in birds and in the Lepi- 
doptera is determined in the same way 
except that the sexes are reversed 
with respect to chromosome composi¬ 
tion. Recent discoveries ( 3, 4, 5, 1®, 
37) have made it seem likely that the 
X-chromosome itself does not deter¬ 
mine sex directly, but that its action 
is due to sex-modifying factors, of 
which the X-chromosome carries a 
disproportionately large and u n - 
balanced number but of which the other 
chromosomes carry a few. This modi¬ 
fication of the originally simple chro¬ 
mosome theory of sex determination 
is of importance chiefly in that it 
offers an explanation of sex intergrades 
and has a bearing on the question of 
the absolute irreversibility of sex. 
The proof of the existence of a 
causal relation between the sex chro¬ 
mosomes and sex determination is so 
strong that biologists almost univer¬ 
sally concede that the sex chromo¬ 
somes are the mechanism which ordi¬ 
narily determines sex. However, it 
may not necessarily follow from this 
that sex can not be reversed by any 
unusual combination whatever of the 
other forces which may have an in¬ 
fluence upon sex. Indeed some experi¬ 
ments ( 9, 10, 18, 34, SB, 36) have been 
interpreted as showing that it is 
occasionally possible to cause an 
individual to change its sex. There is 
still much confusion on this point, 
and until that confusion is cleared up 
the possibility of complete functional 
sex reversal can not be disproved, at 
least among animals like the birds and 
amphibians in which the two sexes do 
not differ greatly with regard to the 
gross anatomy of their organs of 
reproduction. However, very much 
of this confusion is due to a failure to 
distinguish clearly between the primary 
differentiation into two sexes and the 
secondary effects produced by the hor¬ 
mone action of the ovary or testis. 
Several studies have given results 
which conform to the chromosome 
theory of sex determination but in¬ 
dicate that it may be possible to control 
sex by controlling the chromosome 
distribution. One of these is the 
series of studies (81, 32, 33) of chromo¬ 
some behavior in groups like the aphids, 
which reproduce sometimes partheno- 
enetically and sometimes sexually, 
n some species the same aphid may 
produce males or females, or other 
parthenogenetic females like herself 
{38). Apparently there is something 
here governing the chromosome be¬ 
havior, but what it is or to what extent 
it is “internal” and to what extent it is 
a result of the environment (and there¬ 
fore possibly controllable) is not known. 
Riddle’s work {34, 35, 36) with sex in 
pigeons seems to demand the same ex¬ 
planation. He claims to have controlled 
sex in pigeons by the factor of reproduc¬ 
tive overwork—that is, the later eggs 
from females which had been forced to 
* Reference is made by number (italic) to “ Literature cited,” p. 912. 
