Final answer:
Gonads and external genitalia begin to differentiate structurally into male or female forms between weeks 9-12 of fetal development. Testosterone promotes the development of male structures, while its absence leads to the development of female genitalia. The process is dependent on the presence or absence of the SRY gene on the Y chromosome.
Step-by-step explanation:
Development of Gonads and External Genitalia
The development of gonads and external genitalia into male or female structures begins during the fetal period, specifically between weeks 9-12. Initially, embryos have bipotential gonads, which can develop into either male or female reproductive organs, connected to the cloaca via Müllerian and Wolffian ducts. With the presence of testosterone, released from the developing testes (male gonads), the reproductive tissues differentiate into male sexual organs, such as the scrotum and pe_nis. In the absence of testosterone, the bipotential gonads develop into ovaries, and the associated reproductive tissues form structures such as the cl_itoris and labia, which are homologous to the male's pen_is and scrotum.
In males, the scrotum, which houses the testes, is externally located to maintain a temperature conducive to sperm motility and viability. This external positioning allows the temperature around the sperm to be approximately 2 °C lower than body temperature. Conversely, during female fetal development, the bipotential gonads evolve into ovaries and the Müllerian ducts give rise to the uterine tubes and uterus, with the cloaca developing into a vagina, urethra, and rectum.
The primitive gonads begin to form about one month after conception, and the differentiation of reproductive structures continues during fetal development. Females are deemed the 'fundamental' sex, as in the absence of the SRY gene, located on the Y chromosome, development will proceed in a female direction.