258 THE THEORY OF THE GENE 



The Free Martin. 



It has long been known that when twins in cattle are 

 born, one of which is a normal male, the other a "fe- 

 male," the latter is usually sterile. It is known as a free 

 martin. The external genitalia of the free martin are 

 generally female, or much more female-like than male, 

 but it has been demonstrated that the gonads may resem- 



FiG. 141. 



Two enibrvo calves, one of which will become a free martin, whose 



placentas are united. (After Lillie.) 



ble testes. It was shown by Tandler and Keller (1911) 

 that the twins (one of wliich is a free martin) come from 

 two eggs, and Lillie (1917) has fully confirmed this fact. 

 It was also shown by Tandler and Keller that there is 

 present a vascular connection between the two embryos 

 in utero by means of intra-chorionic connections (Fig. 

 141). Magnussen (1918) described a considerable number 

 of free martins of various ages, and has shown by his- 

 tological examination that well-developed testicular-like 

 organs are present in older free martins, i.e., that the 

 characteristic tubular structure of the testes, including 

 rete tubules, sexual cords, and epididymis, is present. 



