2 
SIDNEY I. KOENHAUSER 
were propounded that one ovary gave rise to male-producing 
eggs whereas the opposite ovary gave rise to female-producing 
eggs. Equally valid was the theory that one testis gave rise to 
male-determining spermatozoa and the opposite testis gave rise 
to the female-producing spermatozoa. Two sorts of eggs in 
equal numbers and one type of sperm would give a 50:50 ratio ; 
also two sorts of sperm in equal numbers and one type of egg 
would give a 50:50 ratio. 
These latter ideas are found in modern theories of sex-deter- 
mination, but today they are based on an actual biological founda- 
tion through the use of the microscope and breeding tests. 
2. CHROMOSOMES AND SEX 
Modern theories of sex-determination hold to the first and 
second propositions, stated in the first paragraph of this article. 
If there are two kinds of eggs, male determining and female 
determining, then the sex of the individual is already fixed before 
the sperm nucleus has united with the egg nucleus. At least we 
may say that with the extrusion of the polar cells, the mechanism 
has been brought into play. If there are two kinds of sperm, 
male determining and female. determining, then sex-determination 
depends on the type of sperm uniting with the matured ovum, 
and we may say that sex is determined at the time of fertilization. 
Our present day stand on these questions is based entirely on 
direct observation, both cytological and experimental. In 1902, 
McClung discovered an unpaired chromosome in the testes of 
certain Orthoptera and this chromosome he called a sex-deter- 
miner. This observation, together with the association of this 
chromatic body with sex-determination, was of primary impor- 
tance and it opened up a new era in cytological work. Volumes 
have been written on the mechanism of sex-determination since 
1902, and even at the present time facts on this subject are being 
added almost daily. 
In many groups of animals there is an unpaired chromosome 
in the male, which is called the X-chromosome. This can be 
seen in the somatic cells, in the spermatogonia and in the sperma- 
