The process by which gametes are produced in the gonads is known as gametogenesis.
The process of formation of male gametes or sperm is known as spermatogenesis is of great significance be- cause the gamates are the only link between the generations.
Their nuclear DNA is the depository of all those genetic informations that are needed in the development of new organism.
In monoecious or hermaphroditic animals both male and female gametes are produced in the same individual.
In dioecious animal, the sperm and ova are formed in different individual, but in same species.
ORIGIN OF PGC IN INVERTEBRATE ANIMAL
In invertebrate animal PGC originate in embryonic development. e.g. Ascaris & drosophila.
ORIGIN OF PGC IN VERTEBRATE ANIMAL
In vertebrate animal PGC originated due to separation of germ plasm through somatic plasm.
PGC determined in vertebrates as follows -
1. In fishes - PGC originate by endoderm and establish in developing gonads.
2. In amphibians - Specific cytoplasm present in vegital pole of egg. This cytoplasm form the PGC.
3. In reptiles and aves - PGC originated outside the gonads. eg. in chick embryo PGC originate posterior part of disc and the migrate toward the anterior part and carry through blood circulation in the developing gonads.
4. In mammals - The PGC are formed near the yolk sac from the endoderm (Gonads formed from mesoderm)