76 Bulletin Wisconsin Natural History Society. [Vol. 10, Nos. 1 & 2. 
tilizing the eggs all brought in an x-chromosome and thereby 
caused the production of females. Further investigation may 
show us that this is also the case in the bees and ants. 
Light has been shed on the causes for the development of 
females and males from the parthenogenetic eggs. It has been 
found that in the cases where the unfertilized eggs produce 
females exclusively, one polar body is formed, while in the other 
cases, where males only are produced, two polar bodies appear. 
Castle’s work showed that in the latter case the sex determinant 
is thrown out of the egg in the second polar division, whereas in 
the first case, it is retained within the germ cell. 
Where males as well as females are produced from the par¬ 
thenogenetic eggs, but one polar body is formed. Morgan’s recent 
work on Phylloxera makes it quite certain that in half of these 
unfertilized eggs the sex determinant is thrust out with the forma¬ 
tion of the polar body, while in the other half, the sex chromo¬ 
some is retained. Males are therefore produced in the first case, 
and females in the second. 
In spite of all the criticism that has recently been lodged 
against the chromosome theory of sex determination, still it may 
be stated with certainty, that it is the only plausible hypothesis 
that has given us enough evidence to warrant its acceptance. The 
facts already cited, make it evident that sex in parthenogenetic 
forms can be accounted for on the grounds of such an explana¬ 
tion. There is no positive evidence for the assumption that sex 
is determined by food, temperature, or other environmental fac¬ 
tors surrounding the germ cell. 
I wish to extend my thanks to Professor Michael F. Guyer 
for reading the manuscript and for his many valuable suggestions. 
Zoological Laboratory, The University of Wisconsin. 
May 9, 1912. 
BIBLIOGRAPHY. 
barfurth, d.— Versuche xiber die Verwandlung der Froschlarven. Arch, 
mikr. Anat. XXIX, 1887. 
bataillon, e. —Le Probleme de la fecondation circonscrit par l’impreg- 
nation sans l’amphimixi'e et la partheogenese traumatique.—Arc. 
de Zool. exp. T. 6, ser. 5, 1910. 
