ABSTRACT
In amphibians, formation of gonad takes place during larval stages. The differentiation of gonads into ovaries or testes usually occurs around the time of metamorphosis, although there are some exceptions to this rule. The genetic sex is established at the time of a zygote formation, when the male and female pronuclei of an ovum and a spermatozoon fuse. However, gonads are in undifferentiated stages of development before the first morphological features characteristic of a male or a female gonad are detectable. During that time the somatic cells of a gonad start to differen tiate according to the sex-determining genes and under their influence the germ cells will transform into oogonia or spermatogonia.
