Thereafter, the development of the internal reproductive organs and the external genitalia is determined by hormones produced by certain fetal gonads (ovaries or testes) and the cells' response to them.
The initial appearance of the fetal genitalia (a few weeks after conception) looks basically feminine: a pair of "urogenital folds" with a small protuberance in the middle, and the urethra behind the protuberance.
The other, hidden sex organs are referred to as the secondary sex organs or internal genitalia.
The most important of these are the gonads, a pair of sex organs, specifically the testes in the male or the ovaries in the female.
The sporophyte produces spores by meiosis which in turn develop into gametophytes.
Any sex organs that are produced by the plant will develop on the gametophyte.
The seed plants, which include conifers and flowering plants have small gametophytes that develop inside the pollen grains (male) and the ovule (female).
Sexual reproduction in flowering plants involves the union of the male and female germ cells, sperm and egg cells respectively.
Each planarian transports its excretion to the other planarian, giving and receiving sperm.
The SRY gene, usually located on the Y chromosome and encoding the testis determining factor, determines the direction of this differentiation.
The absence of it allows the gonads to continue to develop into ovaries.
A great variety of genital form and function may therefore be found among animals.
In many other animals a single posterior orifice, called the cloaca, serves as the only opening for the reproductive, digestive, and urinary tracts (if present).