2 
A. D. Darbishire. 
because the conclusions reached in it that the female bears two 
kinds of eggs, $ -producing and 5 -producing, whilst the male 
plays no part in determining sex, is strikingly similar to that arrived 
at by Mendelian analysis. 
Observations on the reproductive bodies of organisms, spores 
and gametes, indicate that although the asexual spores of all the 
higher plants have been shown to be differentiated into two kinds, 
destined to produce male or female gametes; yet up to the present 
only a few organisms, and these animals, have been shown to 
possess morphologically differentiated gametes pre-destined with 
regard to sex. 1 refer to (1) Hydatina, Dinophilus and Phylloxera, 
which have two kinds of ova, large and small, the former destined 
to become female zygotes, the latter male : (2) to Paludina , and 
other molluscs, and possibly some Protozoa, which have dimorphic 
sperms, differing in size, shape and sometimes in the number of 
cilia they bear. 
Observations on the cytological characters of gametes, how¬ 
ever, have disclosed in a large number of cases, striking phenomena 
pointing to complete sex differentiation of the gametes. It has 
been found in a large number of Arthropods (chiefly Insecta, 
Myriapoda and Arachnida) and a few other animals, that one half 
of the sperms differ from the other half in chromosome characters. 
In 1891 Henking discovered that the sperms of Pyrrochoris, a 
hemipteran genus, are dimorphic in chromosome characters, half 
the sperms having one chromosome more than the others. 
MacClung suggested in 1902 that probably this differentiation was 
correlated with sex-determination, but it was left to Wilson and 
Stevens to describe the phenomena more exactly and extend the 
observations to Orthoptera and other Arthropoda. A large 
number of species possessing such dimorphic sperms have now 
been recorded by Wilson and his fellow workers. In all these 
cases it may be said that half the sperms possess something more 
than the other half, i.e., one or more additional chromosomes; 
this additional body has been called the x-element, accessory 
chromosome, or idiochromosome. If several chromosomes con¬ 
stitute the x-element, they are found to behave as one chromo¬ 
some in the cell-divisons. The remaining sperms may only differ 
from the former class of sperms in the absence of the x-element, 
or they may have in addition a y-element in place of the 
x-element, i.e., one or more peculiar chromosomes which differ 
from all the other chromosomes. Thus a species may possess one 
