HORMONES IN DIFFERENTIATION OF SEX 



pendent studies of Lillie (1916, 1917) and 

 Keller and Tandler (1916). The theoretic 

 explanation of the anomaly proposed by 

 these authors was generally accepted and 

 had two far-reaching effects ; it at once pro- 

 vided a simple, functional concept of the na- 

 ture of embryonic sex differentiation which 

 was readily susceptible of experimental test, 

 and it directly stimulated the pioneer ex- 

 periments in the field — the attempt to con- 

 trol sex differentiation in chick embryos by 

 grafting gonad tissue to the chorioallantoic 

 membrane (Minoura, 1921) and the appli- 

 cation of the technique of parabiosis to the 

 problem in amphibian embryos (Burns, 

 1924, 1925). However, as is usually the case 

 with epoch-making theories, the concept of 

 hormonal control of embryonic sex differen- 

 tiation had roots going far back into the 

 past. 



The effects of castration in domestic ani- 

 mals and in humans had been familiar since 

 earliest times, and it was long appreciated 

 that in the vertebrates the gonads are neces- 

 sary for maintaining the structural integrity 

 of the accessory organs of reproduction and 

 for the regulation of their functional acti^vi- 

 ties. It was first clearly demonstrated by 

 Berthold (1849) that this control is exer- 

 cised through the agency of a substance (of 

 a nature as yet unknown) produced in the 

 gonads and carried throughout the body in 

 the circulating blood (for the historical 

 background of this experiment see Forbes, 

 1 949 ) . Thus the conception of a blood-borne 

 agent capable of controlling the growth and 

 activities of distant structures, was estab- 

 lished long before the name hormone was 

 given to such substances. The theory that 

 the differentiation of the genital structures 

 in the embryo is controlled by a hormone, 

 or hormones, produced by the embryonic 

 gonads was a natural outgrowth of this 

 knowledge. This view was first proposed as 

 an hypothesis by Bouin and Ancel in 1903, 

 suggested by the observation of an unusu- 

 ally rich interstitium in the testes of pig 

 embryos during the period of sex differentia- 

 tion. No direct evidence in support of the 

 hypothesis was forthcoming, however, until 

 the hormone theory, in virtually its present 

 outlines, was formulated by Lillie and by 

 Keller and Tandler as an explanation of the 

 freemartin. 



The freemartin^ had been familiar to 

 breeders of cattle for centuries as a sexually 

 abnormal calf, born as twin to a normal 

 male, and its anatomy had been accurately 

 described by John Hunter in the eighteenth 

 century (for references see Lillie, 1917). 

 The external genitalia and mammary glands 

 are typically female in character and the 

 animal was usually regarded as a female, 

 but in rare cases the clitoris may be greatly 

 enlarged and peniform (Numan, 1843, see 

 Lillie, 1917, Fig. 29; Buyse, 1936). Inter- 

 nally, however, elements of the genital tracts 

 of both sexes are present and frequently 

 well developed, and later investigators were 

 often in disagreement as to the primary sex 

 of the creature. The gonads of the freemartin 

 are rudimentary in form but usually show 

 the histologic structure of an abnormal testis 

 which is almost invariably sterile (for an 

 exception see Hay, 1950, and also Hart, 

 cited by Lillie, 1917, page 417). In many 

 cases, however, the gonads are intersexual, 

 showing varying degrees of agenesis of the 

 ovarian cortex associated with rudimentary 

 tubular structures in the medullary or hilar 

 regions (Chapin, 1917; Willier, 1921). Com- 

 monly a well developed male duct system 

 is present, but the development of the fe- 

 male genital tract is variable and in the 

 more modified cases it may be virtually ab- 

 sent. 



It is unnecessary to go into the various 

 lines of evidence which were used to es- 

 tablish the fact that the freemartin is zy- 

 gotically a female (Lillie, 1917, 1923) ; the 

 point has recently been demonstrated cyto- 

 logically by the Barr method (Moore, 

 Graham and Barr, 1957). It will be useful, 

 however, to review the circumstances which 

 pointed to the cause of the anomaly. A free- 

 martin is always associated at birth with a 

 male twin (which is normal) and never with 

 another female; in addition, the dizygotic 

 origin of the pair was demonstrated by 

 Lillie in many cases. Furthermore, for the 

 birth of a freemartin it is necessary that the 

 placentas of the twins be united, with the 

 presence of vascular anastomoses (Fig. 2.1). 

 In the absence of such connections the fe- 

 male twin is always normal. There is a cor- 

 relation between the degree of abnormality 



^ For a discussion of the origin of this name see 

 Forbes (1946). 



