CHAPTER II 



EXPERIMENTAL PRODUCTION OF TWINS 

 IN STARFISHES 



INTRODUCTION 



Since The Biology of Twins was written much new 

 light has been thrown on the nature and causes of 

 twinning and perhaps the clearest analysis of the funda- 

 mentals of twinning has been obtained through the 

 experiments of the present writer on the eggs and 

 embryos of the Pacific Coast starfish. Patina miniata. 

 It was felt that an experimental analysis of twinning 

 was almost impossible in the case of mammals and it was 

 therefore decided to make use of some species whose 

 development was well known to be simple, readily 

 controlled, and capable of easy observation. One 

 would naturally select for such work either the bony 

 fishes or the echinoderms: two groups w^hich for a long 

 time have been favorite materials for experimental 

 biology. The echinoderms were chosen in preference to 

 the fishes partly because the processes of cleavage and 

 gastrulation are simpler and less open to controversy. 

 For a long time I have felt that twinning was intimately 

 associated with the process of gastrulation and on that 

 account it seemed wise to study twinning in a form in 

 which this whole process is so nearly diagrammatic. 



When I went to the Pacific Grove laboratory in the 

 spring of 1920, one of my chief concerns was to study 

 experimentally the process of twinning in some echino- 



