Within the female body, estrone is primarily produced in the ovaries, but is also exuded in smaller amounts by the adrenal glands. In males, estrone and the other estrogens are generated in minor amounts in the testes. Generally, the estrogen most heavily secreted and most active in the body is estradiol, but this substance is soon converted to estrone when it enters the bloodstream. Estriol, on the other hand, is the estrogen primarily produced by the placenta in pregnant women. Together the estrogens, which are typically at their highest levels in the blood during ovulation, stimulate growth of the egg follicle, thicken the wall of the uterus, facilitate muscle contractions, and in lower animals, induce estrus. The hormones are also responsible for breast development and a number of other physical differences between males and females.