INTRODUCTION 



Everyone is or should be interested in twins. It 

 is my task to bring together the facts about twins and 

 to show the bearings of these facts on fundamental 

 problems of biology. It would be an easy task to write 

 a treatise on twins for the expert embryologist or student 

 of genetics, but it is far less easy to present this material 

 adequately and to make it crystal-clear to those who 

 are not specialists. It has seemed necessary to strike 

 a compromise between a technical presentation and 

 a somewhat popular treatment of the subject. Much 

 of the more general matter in all of the chapters will be 

 found available for the general reader, but some of the 

 descriptive embryology, which is the foundation of our 

 special knowledge, will be rather difficult even to the 

 embryologist. Every effort has been made to simplify 

 this part of the book without running the risk of denatur- 

 izing it. Again, there are parts of the chapters on 

 heredity that will appeal especially to students of that 

 important subject, but will have only a minor appeal 

 to the general reader. 



It is impossible to avoid technical terms, especially 

 in descriptive embryology, but where certain simple 

 terms serve as well as the more technical ones they will 

 be used. In referring to early embryos of human beings 

 or of armadillos we might use the words "blastodermic 

 vesicle" or "blastocyst," but it will be simpler to use 

 the common word "egg" for the early mammaUan 

 embryo and its membranes. Again, in speaking of 



