A large number of case are known where sex is modified due to hormones secreted from sex organs. Three classical examples of hormonal or environmental control of sex would be presented here.
Sex in Bonellia
In Bonellia viridis, a marine worm, all larvae are genetically and cytologically similar. In this worm, male individual lives in the uterus of the female (Fig. 17.22). If a particular larva settles near proboscis of an adult female, it becomes a male individual, and lives in the uterus of the female. On the other hand, if it has to develop free in water, it becomes a female. Similarly, if a partly developed male is detached from the proboscis it become an intersex. Obviously the proboscis secretes a substance suppressing femaleness.
The Crew's hen
A case of complete sex reversal was reported in 1923 by Crew, where a fertile female fowl (hen), which had already produced offspring, changed over to a fully fertile male (cock). This resulted due to a damaged ovary in the female. It is believed that ovary in female secreted a male suppressing hormone. Therefore, in absence of ovary, testis could develop.
Incattle, when twins are produced, one of them being male and other female, female is sterile and male is normal. These are known as Free-Martin after the name of the worker, and would be produced only when there is a vascular connection between two embryos. The hormone perhaps suppresses the development of ovary in female.