The sex organs of a green algae Chara are the male antheridia (red) and female archegonia (brown).
A pair of ovaries of Cyprinus carpio (common carp) placed in dissecting dish
The female genitalia of Lepidoptera

The primary sex organs are the gonads, a pair of sex organs, which diverge into testes following male development or into ovaries following female development.

- Sex organ

Gonads start developing as a common primordium (an organ in the earliest stage of development), in the form of gonadal ridges, and only later are differentiated to male or female sex organs.

- Gonad
The sex organs of a green algae Chara are the male antheridia (red) and female archegonia (brown).

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Alpha

Differentiation of the male and female reproductive systems does not occur until the fetal period of development.

Sexual differentiation

Process of development of the sex differences between males and females from an undifferentiated zygote.

Process of development of the sex differences between males and females from an undifferentiated zygote.

Differentiation of the male and female reproductive systems does not occur until the fetal period of development.
The Human Y Chromosome showing the SRY gene which codes for a protein regulating sexual differentiation.

As male and female individuals develop from embryos into mature adults, sex differences at many levels develop, such as genes, chromosomes, gonads, hormones, anatomy, and psyche.

The differentiation of other parts of the body than the sex organ creates the secondary sex characteristics.