FERTILIZATION. EMBRYONIC AND POST-EMBRYONIC DEVELOPMENT

 Fertilization is the fusion of gametes: and ovum and a sperm. When the nuclei of the ovum and sperm combine, they form a zygote.

Two types of fertilization: external and internal.

-External ferttilization. The union of the gametes outside the body of the female. Typical of aquatic animals and some terrestrial animals.

-Internal fertilization. Union of the gametes inside the body of the female. It involves copulation: the transmission of the esperm from the male into the female reproductive system through a copulating organ. It is typical of terrestrial animals and some fish.

Embryonic development

Embryonic development refers to the processes that take place from the formation of the zygote to the birth of the new individual.

Depending on where embryonic development takes place, can be classified into three groups.

Oviparous:

-Embryo develops inside an egg, normally outside the female body.

-Nutrients are provided inside the egg.

-Typical of most birds, reptiles, fish and monotremes.

Viviparous:

-The embryo develops inside the female body.

-Nutrients are provided by the mother.

-Typical of all mammals except the monotremes and some sharks.

Oviviparous:

-The embryo develops inside and egg, inside the body of the female.

-No nutrients are provided by the mother.

-Typical of many sharks, some snakes and lizards, and some insects.

Post-embryonic development

This development stage refers to the processes that take place from birth unitl an individual is capable of reproduction.

Two types of post-embryonic development: direct and indirect.

-Direct development. Offspring are similar in appaerance to the adult, so development simply involves a growth process. Occurs in oviparous animals, reptiles and insects, and viviparous animals such as mammals. In these animals, abundant nutrients are provided in the gg.

-Indirect development. They young, called larvae, go through profound changes until they become adults. Make up the process called metamorphosis. Occurs in animals in wich the egg has few nutrients.

Types of metamorphosis

Simple

-Appaerance of larvae: very similar to the adult.

-Development: continuous with no periods of inactivity. Growth occurs through a series of moults.

-Examples: annelids, molluscs, crustaceans, some insects, echinoderms and amphibians.

Complex

-Appaerance of larvae: very different from the adult.

-Development: discontinuous due to an inactive stage called a pupa or chrysalis.

-Examples: insects, like butterflies amd flies.