392 TRANSACTIONS OF SECTION D. 
building up the deutoplasm or food-yolk. There is good reason for believing that 
the character of this nutrient material will change, during the course of evolu- 
tion, pari passu with the changing character of the organism by which it is 
supplied. Doubtless the change is of a chemical nature, for we know from pre- 
cipitin experiments that the body fluids of closely allied species, or even of the 
two sexes of the same species, do exhibit distinctly recognisable differences in 
chemical composition. It also appears highly probable, if not certain, from such 
experiments as those of Agar upon Simocephalus, that substances taken in with 
the food and which bring about conspicuous modifications of bodily structure, 
may at the same time be absorbed and stored up by the egg-cells so as to bring 
about corresponding changes in the adults into which the eggs develop. 
There seems therefore to be no great difficulty in comprehending, at any rate 
in a general way, how the egg may become the repository of definite chemical 
substances, organ-forming substances if we like to call them so, possibly to be 
classed with the hormones and enzymes, which will influence the development in 
a particular manner as soon as the appropriate conditions arise. 
Unfortunately, time will not allow of our following up this line of thought 
on the present occasion, but we may notice, before passing on, that with the 
accumulation “of organ-forming substances in the egg we have introduced the 
possibility of changes in bodily structure, to whatever cause they may be due, 
being represented by correlated modifications in the germ-cells, and this is doubt- 
less one of the reasons why the germ-cells of different animals are not all alike 
with regard to their potentialities of development.° 
We now come to the question of how the nucleus of the germ-cell acquired its 
great complexity of structure. We are not concerned here with the origin of the 
differentiation into nucleus and cytoplasm and the respective parts played by the 
two in the life of the cell. The problem which we have to consider is the com- 
plication introduced by the sexual process, by the periodically recurring union 
of the germ-cells in pairs, or, as Weismann has termed it, amphimixis. This is 
well known to be essentially a nuclear phenomenon, in which the so-called 
chromatin substance is especially concerned, and it is a phenomenon which must 
have made its appearance at a very early stage of evolution, for it is exhibited 
in essentially the same manner alike in the higher plants and animals and in 
unicellular organisms. 
Let us suppose, for the sake of argument, that when amphimixis first took 
place the chromatin of each germ-cell was homogeneous, but that it differed 
slightly in different germ-cells of the same species as a result of exposure to 
slightly different conditions during its past. history. What would be likely to 
happen when two different samples of chromatin came together in the zygote? 
The result would surely depend upon the interaction of the complex colloidal 
multimolecules of which the chromatin is composed. Various possibilities would 
arise. (1) The two samples might differ in such a way as to act as poisons to one 
another, disturbing each other’s molecular equilibrium to such an extent that 
neither could survive. This is possibly what happens when an ovum is fertilised 
by a spermatozoon of a distinct species, though there are. of course, exceptions. 
(2) They might be so alike as to be able to amalgamate more or less completely, 
so that there would simply be an increase of chromatin of possibly more or less 
modified constitution. (3) They might continue to exist side by side, each 
maintaining its own individual character. 
In the third case the union of the two different samples would give rise to a 
mass of chromatin of twofold nature, and repetition of the process from genera- 
tion to generation would, as Weismann has shown, result in ever-increasing 
heterogeneity, until the chromatin came to consist of a great number of different 
concrete particles, each of which might conceivably differ from all the others. 
But when two heterogeneous masses of chromatin meet in the zygote there may 
be all sorts of mutual attractions and repulsions between the different colloidal 
multimolecules, for all three of our supposed cases may arise simultaneously, 
and thus the results may become extremely complicated. 
The chromatin of the germ-cells in all existing organisms is undoubtedly 
heterogeneous, and this heterogeneity may be to some extent visibly expressed 
5 Compare Cunningham’s ‘Hormone Theory’ of Heredity (Archiv fiir 
Entwicklungsmechanik der Organismen, Bd. xxvi. Heft 3). 
