34 THE PHYSIOLOGY OF TWINNING 



are about as numerous as those bifid at the anterior end 

 alone. 



It is very rare for bifid individuals to be able to 

 emerge from the cocoon, only ten young worms out of 

 4,000 that had emerged being bifid. The forked ends 

 tend to inhibit emergence because the opening of the 

 cocoon is only large enough for a normal worm. Those 

 with deep bifurcations both anteriorly and posteriorly 

 could not possibly escape. The only chance of successful 

 emergence seems to be to start out with the single end 

 first. A few escaped that had a deep bifurcation at one 

 end and a very slight one at the other. Thus the hazards 

 of one-egg twinning seem to be even greater for worms 

 than will be shown to be the case in man. If there 

 were in these worms an inherited tendency to twin, it 

 could not successfully be passed on, for the survivors 

 are far too infrequent to admit of such a character 

 being transmitted in so large a percentage of individuals 

 as actually occur in cocoons. The only alternative 

 conclusion, then, is that the condition is due to environ- 

 mental factors such as those already discussed. 



THE MODES OF TWINNING IN THE OLIGOCHAETES 



The work of Hyman and others has shown that in 

 annelids, and especially in oligochaetes, the head end is at 

 first the only apical end and it is highly susceptible to 

 growth-inhibiting agents. Relatively early in develop- 

 ment, however, the posterior end of the embryo becomes 

 a secondary apical point with a forward-directed gradi- 

 ent. Thus these worms have a double axiate organi- 

 zation with two highly susceptible regions. It is very 

 significant that bilateral doubling (true twinning) occurs 



