96 THE BIOLOGY OF TWINS 



occur, however, seems rather improbable from our 

 knowledge of the early embryology of swine. The field 

 needs further investigation. 



One fact about the twins of both cattle and sheep which 

 places them in a dif event category from armadillo quad- 

 ruplets is that the twins may either both be of the same sex 

 or of opposite sexes. If sex is determined at the time of 

 fertilization, it seems unlikely that twins of opposite 

 sexes could come from one egg. 



Bovine twins may be of four types: (a) two normal 

 males; {b) two normal females; (c) a normal male 

 co-twin with a normal female; {d) a normal male with 

 a freemartin. The exact nature of the freemartin is 

 a question about which there has been great diversity 

 of opinion; it is the freemartin situation which lends 

 special interest to the study of twinning in cattle. 



THE PROBLEM OF SEX IN THE FREEMARTIN 



The freemartin is a sterile twin born co-twin to a 

 normal male. This definition is quite free from implica- 

 tion as to the sex of the sterile individual, and advisedly 

 excludes the so-called ''fertile freemartin." 



John Hunter' (1786) appears to have been the first 

 to study the freemartin and to report an opinion as to 

 its nature. He described three specimens and diagnosed 

 the conditions as follows: The first specimen (seven 

 years old) was apparently a hermaphrodite in that it had 

 a vagina, a rudimentary bicornuate uterus, testes, and 

 vasa deferentia and vesiculae seminales; the second 

 (five years old) was "more like an ox or spayed 

 heifer" in general appearance. It had a vagina with 



^ J. Hunter, London, 1786. 



