A pollen grain, upon reaching the stigma of a flower of the same species, is stimulated to develop by inducing substances present in the stigma.
The pollen forms a long tube, the pollen tube, which grows into the pistil until it reaches the egg. It has a small hole in the integuments, called a microphyll, through which the pollen tube penetrates. Inside the pollen tube are two haploid cells, the sperm nuclei, which are the male gametes.
Inside the egg there is a special haploid cell, the oosphere, which corresponds to the female gamete. The oosphere is strategically located within the egg, right next to the small opening called the mocropile. The pollen tube reaches exactly the ovular micropyle and one of the two sperm nuclei of pollen fertilizes the oosphere, originating the zygote. This will give rise to the embryo.
The other sperm nucleus joins two polar nuclei present inside the egg, giving rise to a triploid tissue, the endosperm, that will nourish the embryo.
The fertilized egg becomes the seed, which contains a small embryo and rests inside. See in more detail: