Excerpt from The Journal of Experimental Zoology, 1921, Vol. 33 The study of the free-martin by F. R. Lilhe ('17 b) has thrown new light on the field of the biology of sex, demonstrating that sexual differentiation is controllable, at least to some extent, by a certain physiological factor. Lillie discovered that in the free-martin, the female of two-sexed twins in cattle, the sex-glands and sex-ducts are modified from the normal female condition to a sterile masculinized state; that in cattle twins an anastomosis of the umbilical blood-vessels exists between the two embryos, and that this anastomosis is responsible for the alteration of the female twin, the sex-hormones from the male twin being thus enabled to gain access to the female twin through the blood current and to produce a profound modification of her reproductive system. A histological study of the same material by Chapin ('17)supported Lillie's conclusion that the free-martin is a modified female. As Lillie suggested ('17 a), "the possibility exists, however, that definitely planned experiments may enable us to regulate time and dosage of hormones better than is done in this experiment of nature; the results of such experiments cannot of course be foreseen... Such experiments will be necessary for the full solution of the stated problem." I therefore, at Professor Lillie's suggestion, undertook to perform such a series of experiments for the purpose of determining whether or not sexual differentiation is experimentally controllable through the secretion of the sex-glands. Gonads were grafted on the membranes of developing chick embryos in relatively young stages; such embryos were then allowed to develop further, and in cases where the grafts grew, the urinogenital systems of the embryos were studied to determine what effects had been produced by the presence of the gonad grafts. The results of these experiments are presented in the present paper and others to appear subsequently. It has already been abundantly proved through numerous studies on birds and mammals that the sex-glands do produce a secretion or secretions which play an important role in the production and maintenance of secondary sexual characters. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.