CONTENTS 



PAGE 



Introduction i 



CHAPTER 



I. Various Kinds of Human Twins .'.... 8 



II. Twinning (Polyembryony) in the Armadillo (Dasy- 



pus novemcinctus) . 25 



III. Modes or Twinning in Other Species of Armadillo 68 



IV. Theories of Polyembryonic Development in 



Dasypus 85 



V. Twinning in Ruminants — ^The Freemartin . . 95 



VI. Twins in Relation to General Biological Prob- 

 lems no 



VII. Variation and Heredity in Twins 125 



Index 181 



IX 



