Choose the correct sequence:

Gametogenesis - fertilisation - zygote formation - embryogenesis.

The correct answer is Option (1) - Gametogenesis - fertilisation - zygote formation - embryogenesis.

Firstly, gametogenesis takes place which means the formation of two types of gametes- male and female. Gametes are haploid cells. If the two gametes are so similar in appearance that it is not possible to categorise them into male and female gametes. They are hence called homogametes (isogametes). However, in a majority of sexually reproducing organisms the gametes produced are of two morphologically distinct types (heterogametes). In such organisms the male gamete is called antherozoid or sperm and the female gamete is called egg or ovum.

 Next step is gametes transfer. Male and female gametes must be physically brought together to facilitate fusion or fertilisation. In a majority of organisms, male gamete is motile, and the female gamete is stationary. Exceptions are a few fungi and algae in which both types of gametes are motile. A large number of the male gametes, however, fail to reach the female gametes. To compensate this loss of male gametes during transport, the number of male gametes produced is several thousand times the number of female gametes produced. The most vital event of sexual reproduction is perhaps the fusion of gametes. This process called syngamy. The term fertilisation is also often used for this process. The terms syngamy and fertilisation are frequently used though, interchangeably.

 Syngamy results in the formation of a diploid zygote. Formation of the diploid zygote is universal in all sexually reproducing organisms. In organisms with external fertilisation, zygote is formed in the external medium (usually water), whereas in those exhibiting internal fertilisation, zygote is formed inside the body of the organism.

 Embryogenesis refers to the process of development of embryo from the zygote. During embryogenesis, zygote undergoes cell division (mitosis) and cell differentiation.