228 AN INTRODUCTION TO MODERN GENETICS 



secondary sex characters is, however, not directly under the control of 

 their genetic constitution but depends on influences proceeding from 

 the gonads. These influences are the well-known sex hormones; a 

 detailed study of their action belongs rather to physiology than to 

 genetics. Here we may mention that there are two theories under con- 

 sideration. According to one, which may be called the classical theory, 

 both hormones are active stimulating agents of secondary sexual 

 diff'erentiation during the later stages of embryonic life. According to 

 the other theory, recently advanced by Wiesner,^ only one of the hor- 

 mones is an active agent in development, namely that of the hetero- 

 gametic sex, male in mammals, female in birds. Diiferentiation towards 

 the organs of the homogametic sex is supposed to go on independently 

 of hormonal stimulation, a reaction to the homogametic hormone being 

 not shown until the organs are fairly fully developed. 



The relation between the hormones and the medullary and cortical 

 (M and F) substances is still not clear. Certainly M and F substances 

 are found in insects, and in insects sex hormonal influences must be 

 weak, since quite small patches of tissue can develop their own sex in 

 mosaics; even in Drosophila, however, there is some inhibitory influ- 

 ence of a gonad on the development of ducts of the opposite sex.^ In the 

 Amphibia, Witschi^ regards the medulla and cortical substances as 

 diff"erent from the male and female hormones, since he has not been 

 able to obtain sex reversal by injection of oestrin. In the chick, on the 

 other hand, it has recently been reported that injection of large quanti- 

 ties of hormone may influence the earliest stages of gonad development.^ 

 In mammals, hormones certainly affect the development of the gonads 

 in fairly late stages (at birth and after). In earlier stages the position is 

 not so clear; in unlike-sexed twins with joined placental circulations in 

 cattle, the female gonad is transformed into a testis, the resulting 

 intersex being known as a freemartin, but there is no proof that the M 

 substance responsible for this is the same as the adult testicular hor- 

 mone. On the other hand, even male embryos contain large quantities 

 of oestrin derived from the mother, which has no effect on their sexual 

 development, but it is probable that this oestrin is inactivated by 

 combination with protein. 



^ Wiesner 1935. 



' Cf. Bridges 1932. For an interpretation of Lymantria gonads in terms of 

 cortex and medulla, see Sato 1932. 



^ Witschi 1937. In more recent work (unpubl.) positive results were obtained. 

 * Willier, Gallacher and Koch 1937. 



