609 
1908-9.] Mendelian Action on Differentiated Sex. 
as fertilisation is completed, I speak of a male zygote and a female zygote. 
The proof for each of these statements will come up presently. 
I further employ Weismann’s terminology and views to a certain extent. 
Weismann has shown that the development - results in the adult (its 
determinates) are represented causally in the gametes and zygote by 
“ determinants,” and that there is continuity of the germ plasma. His 
term “ id ” for the group of determinants necessary in the gametes or zygote 
to produce a holophyte I also find convenient. Mendel’s and Weismann’s 
terminologies enable us to state and discuss this question — a most important 
point — and the terminology must be considered as analogous to an algebraic 
one. In the human race sex has differentiated. The zygote necessarily 
contains the determinants for the dominant (potent) and recessive (non- 
potent) sexual determinants, and this makes it an impure dominant, as in 
F 1 , i.e. it contains D and It determinants combined, and never normally 
segregated, I hold, in subsequent development. The human zygote being an 
impure dominant and produced by the union of male and female gametes, 
we have to consider the number and natures of the gametes producing such a 
zygote. For long it has been considered that one gamete from each parent 
was necessary, but the existence of dimorphic male and female gametes has 
shaken this belief. Many of the animals lower in the scale than man have 
two kinds of male gametes, one of them having in insects an extra chromo- 
some (McClung), and some have dimorphic female gametes. In man there 
are dimorphic spermatozoa, but it is disputed as to whether the ova are 
dimorphic (Russo, Heape). 
It has also long been held that sex was not determined at fertilisation, 
but later in the development of the embryo. This, however, is erroneous, 
and there is no evidence that the human zygote is hermaphrodite or 
indifferent in sex at any early stage. 
It has been urged by some able observers (Beard, Castle, and many 
others) that there are male and female eggs : this, however, makes an 
organism hermaphrodite, and the male gamete contain no sexual 
determinants, and is incompatible with the true view as to the origin of the 
gametes. 
For reasons that will presently appear, I consider there is the highest 
probability that in human fertilisation a sex and non-sex male gamete, and 
a sex and non-sex ovum are concerned : that for the formation of the male 
zygote a sex male gamete and non-sex female gamete have united ; for the 
formation of the female zygote a sex female gamete and non-sex male 
gamete. This makes sex determined at fertilisation, and necessarily gives 
the 50 per cent, of the two sexes. 
VOL. XXIX. 
39 
