26 THE PHYSIOLOGY OF TWINNING 



toward it as it would be if this individual had control 

 of its own gradient. 



An even more complete domination of a weaker 

 individual by a stronger occurs when one of a pair of 

 archentera arising from one continuous basal area is 

 distinctly smaller than the other. I have watched 



r8 



Figs. 18-21. — Four outline figures of a single Patiria larva, drawn 

 on four successive days, to show the way in which a twinned larva not 

 infrequently returns to an almost normal single or untwinned condition. 

 The smaller twin archenteron becomes a sort of parasite upon the side 

 of the larger. See text for details. (Original.) 



from day to day all of the stages of such a process. The 

 series of drawings (Figs. 18, 19, 20, 21) were made 

 from a single individual at intervals of about twenty-four 

 hours. It is remarkable how nearly like a single nor- 

 mal individual such a twin can become after absorbing 

 its weaker brother, though neither one of them is truly 

 primary in the sense that it represents the original 

 individual, nor secondary in the sense that one is a 

 bud from the other. This finding has a very definite 

 bearing on theories of the origin of double monstrosities 

 in the higher animals, especially in vertebrates. There 

 is frequently a marked difference in the size and degree 

 of normality in the two components in double monsters 

 and it frequently happens that, as in the starfish cases 



