EXPERIMENTAL PRODUCTION OF TWINS 25 



by the anterior ends of their archentera, making a type 

 of embryo with one anterior end and a divided posterior 

 like the type of double monster known technically as 

 katadidymus (Fig. 5). 



THE INFLUENCES OF ONE TWIN UPON ANOTHER 



One of the most significant features of twinning in 

 the starfish has to do with the apparent control one 

 twin component has over another. When, as in cases 

 such as those shown in Figures i and 2, one of the 

 components is distinctly the primary individual and 

 the other is secondary, only the primary archenteron 

 is able to differentiate in normal fashion. There are 

 numerous instances in which the primary archenteron 

 produces its coelomic pouches and breaks through a 

 mouth, while the secondary archenteron either closes the 

 blastopore and becomes a cyst or else remains in an 

 undifferentiated condition. In such cases it seems that 

 the primary component must in some way exercise an 

 inhibiting influence upon the secondary component. 

 Just what may be the mechanism of such an inhibition 

 we do not know for sure, but it seems highly probable 

 that it is a phenomenon involving the exercise of domi- 

 nance and subordination through the gradient. It is 

 probable that the gradient of the primary individual, 

 on meeting that of the secondary, tends to overwhelm 

 the latter and reverse its direction, thus making it a 

 subordinate part of the primary gradient. That this is 

 more than a mere conjecture is evidenced by the fact 

 that in a specimen like that shown in Figure i the direc- 

 tion of ciHary beat in the secondary component is at 

 least mainly away from its own blastopore, instead of 



