TWINNING IN EARTHWORMS 35 



most frequently and is most complete in the two apical 

 regions of the double gradient and that the part of the 

 body that most frequently fails to undergo twinning is 

 that region which represents the common basal region, 

 or region of lowest metabolic rate, of the two gradients. 

 It is also important for us to note that such bilateral 

 organisms as the worms show almost exactly the same 

 types of double monstrosities as have been described 

 for the vertebrates. The explanation of the fact that 

 heads and tails are double and the middle of the body 

 single is that the free ends have undergone bifurcation 

 or longitudinal fission. There is not even the shadow 

 of suggestion that the double monsters arose as products 

 of the fusion of two individuals arising from separate 

 embryonic axes, for there is really no possibility that 

 such a thing could occur in these worms. The fact 

 that all sorts of double monsters, in these simple, bilateral, 

 metameric organisms actually do occur by dichotomy 

 of the apical ends goes far to support our theory of the 

 origin or conjoined twins in the vertebrates. It should 

 also be said that every degree of bifurcation is present, 

 from a very slight terminal broadening to a very deep 

 bifurcation running nearly half the length of the worm. 

 This situation also entirely parallels the condition seen 

 in the vertebrates and shows that individuals vary 

 greatly in their responses to the factors that induce 

 twinning. Another situation in these double worms 

 that parallels that in vertebrate double monsters is 

 that the portion of the body that is apparently single 

 is in reality nearly all double. Thus there are usually 

 two complete nerve cords in the united part of the body 

 and they are 180° apart; which means that the ventral 



