Biology XII:1 Reproduction in Organism(4 Sexual Reproduction)

Sexual Reproduction: Post-Fertilisation Events Events taking place after fertilisation are called post-fertilisation events. Zygote The haploid gametes fuse to form a diploid zygote in all organisms. In external fertilisation, a zygote is formed in an external medium, and in internal fertilisation, a zygote is formed inside the individual. The development of a zygote depends upon the life cycle of an organism and its surroundings. In some organisms, the zygote does not develop immediately, and develops a thick wall around itself. This wall is resistant to damage and desiccation. Embryogenesis It is the process of development of the embryo from the zygote. The zygote undergoes cell division and differentiation. Cell division increases the number of cells of the embryo, and cell differentiation helps the cells undergo modifications to form specialised tissues and organs. Animals can be grouped into two categories based on how and where the development of the zygote takes place. These categories are: Oviparous − The fertilised egg is covered by a calcareous shell and is released into the outside environment. The development takes place inside the egg and the young one hatches out (example: birds and reptiles). Viviparous − The development of the zygote takes place inside the female body, and the developed young one is delivered outside (example: mammals, including humans). In flowering plants, the zygote is formed inside the ovule. Zygote → Develops into → Embryo Ovule → Develops into → Seed Ovary → Develops into → Fruit → Contains → Seeds → Disperse and germinate to form new plants