A generalized sexual reproduction in organism has 3 events:
1. Pre-fertilization events: A number of preparatory events are required to facilitate the fusion of gametes to give zygote. These preparatory events are as follows:  

A. Gametogenesis: The maturation and formation of gamets inside the sex organ is known as gametogenesis. The gamates are of two types; male and female. In most of the organism gamates are haploid (1N) cells with single set of chromosomes. In lower plants such as algae, both gametes are identical in shape, size and activity. These kind of gametes are known as isogametes or homogametes (Figure 20.2). In sexual reproduction, it is a generalized fact that gametes from male and female are morphologically distinct, these type of gametes are known as heterogametes. In these organisms, male gametes are known as microgametes or spermetazoa, and the female gamete termed megagametes or ova. Male gametes are small, motile and approach female gametes for fertilization. In contrast, female gametes are large, non-motile and store food material to provide nourishment for developing embryo.