EXPERIMENTAL PRODUCTION OF TWINS 27 



just described, the larger more or less completely absorbs 

 or grows over the smaller. The larger component has 

 come to be called the "autosite" and the smaller, the 

 ' 'parasite. ' ' The genetic relationship between the two has 

 been discussed by Stockard, who considers the parasite 

 component as a lateral bud derived from the autosite 

 and kept in subordination to the latter much as a lateral 

 plant bud is inhibited by the growth of the main growing- 

 point. That such an explanation as this is inapplicable 

 to the condition in the starfish need hardly be -stated. 



CONCLUSIONS 



The reader will now have become aware that in the 

 simple development of the starfish there appear, dia- 

 grammatically almost, practically all of the various 

 phenomena that are associated with one-egg twinning. 

 We have been able to observe all of the stages of develop- 

 ment and to see many of the events of the twinning 

 process. We know, moreover, to a considerable extent 

 the causes of the various twinning processes observed. 

 If we are to understand the more intricate twinning 

 phenomena in the armadillo, in man, in the fishes, and 

 in various other animals or even plants, it seems clear 

 that we shall have to refer these conditions back to those 

 of the starfish as a sort of norm, and it is my present 

 opinion that the starfish situation throws much light on 

 the whole problem of the physiology of twinning. 

 I therefore scarcely need to offer an apology for giving 

 first place in the present volume to the lowly starfish 

 and for relegating to less prominent positions those 

 phases of twinning that are more familiar and that are 

 more imbued with human interest. 



