The results of the previously described experiments induced further
experiments, the results of which appear fitted to afford some
conclusions as regards the composition of the egg and pollen cells of
hybrids. An important matter for consideration is afforded in _Pisum_
by the circumstance that among the progeny of the hybrids constant
forms appear, and that this occurs, too, in all combinations of the
associated characters. So far as experience goes, we find it in every
case confirmed that constant progeny can only be formed when the egg
cells and the fertilising pollen are of like character, so that both
are provided with the material for creating quite similar individuals,
as is the case with the normal fertilisation of pure species[38]. We
must therefore regard it as essential that exactly similar factors are
at work also in the production of the constant forms in the hybrid
plants. Since the various constant forms are produced in _one_ plant,
or even in _one_ flower of a plant, the conclusion appears logical
that in the ovaries of the hybrids there are formed as many sorts of
egg cells, and in the anthers as many sorts of pollen cells, as there
are possible constant combination forms, and that these egg and pollen
cells agree in their internal composition with those of the separate
forms.
[38] [“False hybridism” was of course unknown to Mendel.]
In point of fact it is possible to demonstrate theoretically that
this hypothesis would fully suffice to account for the development of
the hybrids in the separate generations, if we might at the same time
assume that the various kinds of egg and pollen cells were formed in
the hybrids on the average in equal numbers[39].
[39] [This and the preceding paragraph contain the essence of the
Mendelian principles of heredity.]
In order to bring these assumptions to an experimental proof, the
following experiments were designed. Two forms which were constantly
different in the form of the seed and the colour of the albumen were
united by fertilisation.
If the differentiating characters are again indicated as _A_, _B_, _a_,
_b_, we have:
_AB_, seed parent; _ab_, pollen parent;
_A_, form round; _a_, form angular;
_B_, albumen yellow. _b_, albumen green.
The artificially fertilised seeds were sown together with several seeds
of both original stocks, and the most vigorous examples were chosen for
the reciprocal crossing. There were fertilised:
1. The hybrids with the pollen of _AB_.
2. The hybrids " " _ab_.
3. _AB_ " " the hybrids.
4. _ab_ " " the hybrids.
For each of these four experiments the whole of the flowers on three
plants were fertilised. If the above theory be correct, there must be
developed on the hybrids egg and pollen cells of the forms _AB_, _Ab_,
_aB_, _ab_, and there would be combined:--