TWINNING IN EARTHWORMS 37 



ends and the ventral surface, and is less complete at 

 regions of lower rate of metabolism, the middle parts of 

 the body and the dorsal side. In other words, inhibition 

 strikes the apical points of the axis of polarity and the 

 axis of symmetry, just as our general theory demands. 



The type of twinning that prevails among the 

 annelids is bilateral twinning or isolation of the right 

 and left side of the single embryonic axis. This type of 

 twinning was only occasionally met with in the echino- 

 derms, where the prevailing mode is one which involves 

 double or triple gastrulation, or the formation of paired 

 embryonic axes. Since both of these modes of twinning 

 are common throughout the animal kingdom, it seems 

 well to have shown them in unequivocal form in groups 

 where the processes are clear and the causes known at 

 least to a high degree of probability. It is my behef 

 that the analysis of the double-monster situation in the 

 earthworms practically settles the controversy as to 

 the mode of origin of vertebrate conjoined twins, and 

 this chapter is therefore placed well toward the beginning 

 of the book because of its bearing on the equivalent 

 conditions in the vertebrates about which there have been 

 widely divergent opinions. 



