TRANSACTIONS OF SECTION I. 671 
much in male and female, have two very obvious external effects: (1) The 
external colour of the male is redder than the female; (2) the male attains a 
larger size than the female. These two characters of the male are associated 
with the formation of tetronerythrin in excess of lutein, and of glycogen in 
excess of fat. 
In crabs of both sexes infected with Sacculina we do not find any of these 
cyclical changes taking place, because such crabs neither grow, moult, nor repro- 
duce. In them we find a remarkably constant internal condition, viz., low 
percentage of glycogen (about 0°5 per cent.) and a constantly high percentage of 
fat (13 per cent.). The high percentage ot fat is associated with the fact that 
the Sacculina roots absorb large quantities of fat from the crab, which meets 
the demand by an extra supply. The internal condition of Sacculinised crabs 
resembles, on the whole, that of female crabs with mature ovaries, except that 
there is little or no Jutein in the blood. The livers of Sacculinised crabs are, 
however, always yellow with lutein, showing that this substance is formed in 
large quantities, and perhaps it is seized on by the Sacculina roots so rapidly 
that it does not appear in the blood. 
(iii) The Physiology of Sex Determination. By Dr. L. Doncaster. 
Mr. Geoffrey Smith has shown that the metabolism of fat and other sub- 
stances is recognisably different in male and female crabs, and that male crabs 
infected with Sacculina tend to have a type of metabolism resembling that of 
the normal female. Such parasitised males assume more or less completely the 
female sexual secondary characters, and may go so far as to produce ova in 
their testes. Somewhat similar phenomena with regard to metabolism have been 
shown by ‘Steche to exist in Lepidoptera (‘ Zeitschr. f. indukt. Abstamm.’ 8, 1912. 
p- 284). In the species which he studied the female has green blood, the male 
yellow, owing to the fact that chlorophyll, or an immediate derivative of it, is 
present in the blood of the female. The blood of the female differs also in 
other respects from that of the male, so that he says that there is more difference 
in the metabolism of the two sexes of the same species than in that of the same 
sex of different species. i 
Now breeding experiments with insects and vertebrates have shown appar- 
ently conclusively that the determination of sex is due to an inherited factor 
which is present in one sex and absent in the other; that is to say, that one sex 
is homozygous in respect of this factor, the other sex heterozygous. But a 
difficulty arises from the fact that in some groups the female appears to be 
heterozygous, in others the male. It has been suggested that since in crabs the 
male may assume the female sex-characters under the influence of Sacculina, in 
this case it is the male which is heterozygous for sex. I wish to put forward 
a somewhat different suggestion, which would also help to get over the difficulty 
that although it seems certain that sex is normally determined by an inherited 
factor, yet there is a good deal of evidence that it may be altered by environ- 
mental influences in the embryo, or even in later life, as in the case of the infected 
crab. I suggest that all individuals, both of animals and plants, contain 
potentially the factors for the characters of both sexes—that all are, in fact, 
potential hermaphrodites—but that in addition each individual either receives 
or does not receive a sex-determining factor, a factor which determines which set 
of sexual characters shall appear. This determining factor, which is very 
probably borne in a particular chromosome, does not introduce into the zygote 
the characters associated with a particular sex—say the female—for these by 
hypothesis are present in every zygote, but it causes them, rather than the male 
characters, to develop. The characters of the corresponding sex are ‘ ausgelést,’ 
as the Germans would say, rather than introduced by the sex-determiner. And 
the way it does this, on my hypothesis, is by determining a particular form of 
metabolism. The presence of this factor causes a particular metabolism, and 
this metabolism causes the female characters to appear; in its absence the meta- 
bolism would have been different and the male characters would have appeared. 
I suggest, in fact, that the type of metabolism determines tne sex, and not the 
sex the type of metabolism. If, then, in a male crab the Sacculina induces the 
type of metabolism characteristic of the female, it is not surprising that female 
characters appear, even though the crab may have received no inherited female 
