REVIEW OF BIOLOGY OF SEX-DETERMINATION 
5 
some. Very often it lags behind the anaphase autosomes in 
either the first or second spermatocyte division. The Y-chromo- 
some likewise often fails to form a leptotene thread. In the 
growth of the spermatocyte, and in the maturation divisions, 
the X- and Y-chromosomes show considerable variation in degree 
of association. In the primary spermatocyte division of many 
Heteroptera the X- and Y-chromosomes divide independently. 
They then come into contact and separate reductionally in the 
second spermatocyte division. 
We may now ask how the sex chromosomes are related to the 
autosomes. That the X-chromosome bears many genes for 
characters having nothing to do with the process of sex is known 
from breeding experiments. In the female the X-chromosomes, 
except where there are differences in size, cannot be distinguished 
from ordinary autosomes. Recent experiments, especially those 
of Bridges on Drosophila, indicate very clearly that there are 
specific sex-genes in the X-chromosome, which working in con- 
junction with the genes of the autosomes are capable of producing 
males or females or even intermediates in cases where the 
normal relationship is upset. 
We are therefore entirely rid of the older idea that the X- 
chromosome is composed of a different kind of chromatin from 
that found in the autosomes and that the sex of the zygote de- 
pends upon the amount of X-chromatin it receives. Sex is now 
put upon a basis of specific genes. 
The Y-chromosome until recently has not been known to 
carry specific genes for bodily characters. Indeed, this chromo- 
some is generally regarded as merely a degenerate X-chromosome 
which has lost its sex-genes and most of its other genes as well. 
That it is essential to normal development in species ordinarily 
having it present was shown in the non-disjunction experiments 
of Bridges. Thus, male Drosophila without a Y-chromosome 
are sterile. In size the Y-chromosome may be as large as the X or 
it may be almost insignificant in comparison to it. In Enchenopa 
hinotata, studied by the author, both the X and Y elements form 
threads in the leptotene stage of the primary spermatocytes. 
These threads are thicker than those of the autosomes and never 
