INTRODUCTION 7 



Siamese type and so-called '' parasite" twins are also 

 oddities that have long been known but little under- 

 stood. Owing to the special interest that naturally 

 attaches to human affairs, the first chapter will be 

 devoted to human twins, although so little is known 

 about their mode of development. The second chapter 

 gives a full account of the process of polyembryonic 

 twinning in the armadillo, Dasypus novemcinctus , and 

 there is reason to believe that in this material we have 

 the only key to the mechanics of human twinning. 



I wish to express my thanks to my friends W. H. 

 Osgoode and F. R. Lillie for reading the manuscript 

 and for useful suggestions; to Mr. Kenji Toda for his 

 aid in preparing illustrations; and to those authors 

 from whom figures have been borrowed. 



