Im zweiten Jahre wurde für die weitere Befruchtung die Hybride Aab als Samenpflanze, die andere aBb als Pollenpflanze verwendet.
In the second year, the hybrid Aab was utilized as seed plant, and the other, aBb, as pollen plant for further fertilisation.
further fertilisation = die weitere Befruchtung Mendel uses the singular with a definite article here. Note that unlike his previous experiments, Mendel does not leave the hybrids to self-fertilisation in this experiment. What he does is construct two different “hybrids”, which notably are “hybrid” with respect to one of the two respective characters only, in a first step of artificial cross-pollinations, and then using these for another set of artificial cross-pollinations. The first step is preparatory in nature, both in the sense of preparing the experiment and in the sense of constructing its elements, while the second step constitutes the crucial experimental intervention, in analogy to the crossing of “pure species” in preceding sections.