1 75 
On the Nature of Fertilization and Sex, 
Now the zygote is built up of the union of two cells of 
physiologically different nature. The reactions that occur between 
the protoplasts of these two cells have been shown to resemble in 
many ways those manifested between parasite and host or between 
the two members of a symbiotic alliance. Can the zygote be 
included among such cases as those just cited? Is there any 
evidence that the zygote (using the term to include the product 
resulting from the development of the fertilized egg-cell) is in 
reality a dual structure, and is it possible to recognise the two 
“partners” forming it, after union has occurred at fertilization ? 
From cytological evidence it appears that the maternal and 
paternal components of the nucleus may retain their identity, 
as manifested by the behaviour of the chromosomes, although 
both are in the same “ cell.” Among the copepods, for example, 
the maternal and paternal chromosomes may retain their own 
spindles for many successive nuclear divisions and the dual nature 
of the cell structure be very evident. 
One may say then that the dual nature of the fusion-product 
is not, of necessity, immediately and entirely obscured, though 
more evident in some cases than in others. The cytological 
evidence is not incompatible with the view that the zygote, at 
least during the early stages of development, is to be regarded as 
a dual organism, the constitution of which is analogous to that of 
a symbiotic partnership in which the cells of the two partners are 
not delimited from one another by cell walls as are the cells of 
plants, and the association between whose protoplasts is extremely 
intimate. 
If the zygote may be so regarded during the early stages 
of its development, there are no logical grounds for refusing to 
extend this conception to the whole of the life-history, though 
the degree of intimacy between the two components may 
vary at different times. It is significant that the behaviour of the 
chromosomes during meiosis suggests that the nucleus is a double 
structure; and that cytologists in general agree that the “ individu¬ 
ality” of the chromosomes—and therefore presumably the double 
nature of the nucleus—is maintained during the so-called resting 
phase of the nucleus. The assumption that the cells of the zygote 
remain double in structure is also involved in an acceptance of the 
Mendelian scheme of inheritance. 
To regard the cells of the zygote as dual in nature is thus 
not unorthodox. 
