Syngamy - Patterns of Sexual Reproduction
Sperm fuses with the egg. This results in both the union of the paternal nucleus with the maternal one (karyogamy), as well as the fusion of the cytoplasms of the two gametes (plasmogamy). Syngamy leads to fertilisation producing a zygote which develops into new individuals, depending upon the size and shape of the gametes involved, syngamy can be subdivided into three types.
i) Isogamy: The gametes are morphologically similar although they may differ in their physiological and biochemical properties. For example, the gametes produced from the male and female gametocytes of Monocystis.
ii) Anisogamy: The gametes differ in size and structure and are collectively known as anisogametes. Of these, the smaller ones are usually more numerous and motile. They are called the male gametes (or the micro-gametes as in protozoans and the sperms as in metazoans). The fusion of micro - and macro-gametes is known as anisogamy. It is frequently found in protozoans as in Plasmodium and Vorticella. In higher phyla the term fertilisation is used instead of anisogamy.
iii) Oogamy: In oogamy one gametes type is always motile and usually small (the sperm) and the other is always nonmotile and large (the egg). All metazoans exhibit oogamy. The eggs of most fully terrestrial non-chordates such as insects have shelled eggs. The shell bears a minute pore (micropyle) for allowing the entry of sperms for fertilization.