discovered in the walls of the intestine and other organs. Here, however, we are more especially concerned with the gonads and other reproductive organs. A great deal of evidence has now been obtained that the influence of the testes and ovaries on secondary sexual characters is due to a hormone formed in the gonads and passing in the blood in the course of the circulation to the organs and tissues which constitute those characters. The fact that transplanted portions of testes in birds (cocks and drakes) are sufficient to maintain the secondary characters in the same condition as in normal individuals shows that the nexus between the primary and somatic organs is of a liquid chemical nature and not anatomical, through the nervous system for example. Many physiologists in recent years have maintained that the testicular hormone is not derived from the male germ-cells or spermatocytes, but from certain cells between the spermatic tubuli which are known as interstitial cells, or collectively as the interstitial gland. The views of Ancel and Bouin, [Footnote: _C. R. Soc. Biol., iv._] published in 1903, may be described in large part as theory. They state that the interstitial cells appear in the male embryo before the gametocytes present distinctive sex-characters. They conclude that the interstitial cells supply a nutritive material (hormone?), which has an effect on the sexual orientation of the primitive generative cells. In addition to this function, the interstitial cells by their hormone also give the sexual character to the soma. When castration is carried out at birth the male somatic characters do not entirely disappear, because the hormone of the interstitial cells has acted during intrauterine life. The functional independence between the interstitial cells and the seminal tubules is shown by the fact that if the vasa deferentia are closed the seminal gland (_i.e._ tubules) degenerates while the interstitial cells do not. In the embryo the interstitial gland is large, in the adult proportionately small.