THE LIMITS OF HEREDITY 199 



and uniovular twins in mankind is strong and un- 

 equivocal.* 



It was formerly supposed that identical twins were 

 produced by separation of the first two cells formed by 

 the division of the fertilised egg, the first two cells into 

 which the egg divides becoming separated after the egg 

 has been fertilised by a single sperm. There is experi- 

 mental evidence of the production of two embryos 

 or of double monsters from invertebrate eggs by this 

 method. But since the investigations of Newman 

 and Patterson on the armadillo, in which the latter 

 showed that the four embryos at a birth are produced 

 by budding at an early stage of development, it has 

 seemed much more probable that identical twins 

 (surrounded by a common chorion) in man are pro- 

 duced in a similar way. Moreover, the number of 

 buds in the nine-banded armadillo is occasionally 

 three or five, and varies more widely in other species ; 

 and probabl}^ in man also the number is not abso- 

 lutely fixed, but may occasionally be three, thus 

 accounting for some cases of triplets of the same sex. 



Zeleny (1921) finds a definite relation between the 

 number of twin births and triplet births in mankind. 



Thus, if - is the number of twin births, then the 

 n 



number of triplet births will very nearly approximate 

 2- In Prussia during the years 1826-49 the 



ft' 



* A recent case (Arey, 1921) proves directly the origin of 

 identical twins from a single ovum. Twin embryos are described, 

 each 12*3 mm. long, and contained in a single amnion and chorion. 

 The two umbilical cords and yolk-stalks were inserted in a common 

 yolk-sac. In a second pair of monochorionic twin embryos with 

 separate amnions, one of the embryos had neither yolk-stalk nor 

 sac. It is suggested that the human embryo probably shows 

 rather rigid determinate cleavage, and that monozygotic or 

 identical twins result not from the separation of blastomeres or 

 blastomere clusters in early cleavage, but " from later fission of 

 the inner cell mass." 



