i8 



THE PHYSIOLOGY OF TWINNING 



primary and the other the secondary component. Such 

 twin larvae develop symmetrically throughout their 

 lives and reach fairly advanced stages. Typical examples 

 of the relations of the two components are shown in 



Figs. 4-6. — Three very common types of symmetrical twin larvae 

 of Patiria miniata. Scores of such twins were found in almost every 

 batch of larvae which were in any way retarded in development. 

 (Original.) 



Figures 3, 4, 5, 6, 7. When in larvae of this sort one 

 of the archentera is originally smaller or secondarily 

 comes to be less active in its growth than the other, it 

 quite commonly happens that the smaller one comes into 

 contact with the larger, fuses with it at its anterior 

 end, and, after closing its blastopore, becomes first a 

 pouch of the larger archenteron and then merely a 

 thickening in the wall of the latter. This is a very 



