THE FREE-MARTIN; A STUDY OF THE ACTION OF 

 SEX HORMONES IN THE FOETAL LIFE OF CATTLE 



FRANK R. LILLIE 



Zoological Laboratory, University oj Chicago 



TWENTY-NINE FIGURES 



I. INTRODUCTION 



In a preliminary paper (Lillie '16) the author has briefly dis- 

 cussed the theory of the sterihty which is the rule, subject to a 

 few exceptions, in the female of two-sexed twins of cattle. In 

 such females, commonly known as free-martins, the internal 

 organs of reproduction are usually predominantly male in 

 character, and the external organs are usually, at least, of the 

 female type; there are however considerable variations as will 

 appear. The conclusion was reached that the sterile free-martin 

 is zygotically a female, modified by the sex hormones of the male 

 twin, which circulate in both individuals during foetal life owing 

 to secondary fusion of the chorions and anastomosis of the foetal 

 circulation of the two individuals. The conditions thus estab- 

 lished in this natural experiment enable us to draw far-reaching 

 conclusions as to the origin and the differentiation of sex-char- 

 acters in mammals. A somewhat extended treatment of the 

 data, and analysis of the facts is therefore herewith presented. 



Previous studies have dealt mainly wdth the anatomy of the 

 free-martin and have furnished important, though incomplete 

 data on this part of the subject. These will be considered in 

 their appropriate connection. A study of L. J. Cole's ('16) sup- 

 plies som€ important statistical data referred to beyond. We 

 shall introduce the subject by a consideration of the various 

 theoretical points of view from which the discussion must start. 

 In considering these it will be important to define more precisely 

 the condition with which we are dealing. The phenomenon of 

 sterility of the female of different sexed twins is usually consid- 



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