42 PROBLEMS OF GENETICS 



twin as ordinarily used refers to the simultaneous birth of two 

 individuals. Those who are naturalists know that such twins 

 are of two kinds, (i) twins that are not more alike than any other 

 two members of the same family, and (2) twins that are so much 

 alike that even intimate friends mistake them. These latter 

 twins, except in imaginative literature, are always of the same 

 sex. 



It is scarcely necessary for me to repeat the evidence from 

 which it has been concluded that without doubt such twins arise 

 by division of the same fertilised ovum. There is a perfect 

 series of gradations connecting them with the various forms of 

 double monsters united by homologous parts. They have been 

 shown several times to be enclosed in the same chorion, and the 

 proofs of experimental embryology show that in several animals 

 by the separation of the two first hemispheres of a dividing egg 

 twins can be produced. Lastly we have recently had the ex- 

 traordinarily interesting demonstration of Loeb, to which I may 

 specially refer. Herbst some years ago found that in sea water, 

 from which all lime salts had been removed, the segments of the 

 living egg fall apart as they are formed. Using this method 

 Loeb has shown that a temporary immersion in lime-free sea 

 water may result in the production of 90 per cent, of twins. 

 We are therefore safe in regarding the homologous or "identical" 

 twins as resulting from the divisions of one fertilised egg, while 

 the non-identical or "fraternal" twins, as they are called, arise 

 by the fertilisation of two separate ova. 3 



In the resemblance of identical twins we have an extreme case 



3 These fraternal twins, which show no special resemblance to each other, 

 are like the multiple births of other animals, and there is no disposition for them to 

 be of the same sex. In the sheep, for example, statistics show that the frequency 

 of pairs of twins, male and female, is approximately double that of the frequency 

 of pairs, both male or both female, as it should be if the sex-distribution were for- 

 tuitous. For instance Bernadin (La Bergerie de Rambouillet, 1890, p. 100) gives 

 the following figures for twin- lambs in Merinos: both male, 87; both female, 83; 

 sexes mixed, 187. The 9-banded Armadillo (Dasypus novemcinctus) , in which 

 the young born in one litter are said to be always of one sex, is the only known 

 exception in Vertebrates, and is presumably a genuine case of normal polyembryony 

 (see especially, Rosner, Bull. Ac. Soc. Cracovie, 1901, p. 443, and Newman and 

 Patterson, Biol. Bull., XVII, 1909, p. 181, and an important paper lately published 

 by H. H. Newman and J. T. Patterson, Jour. Morph., 1911, XXII, p. 855. 



