32 



THE PHYSIOLOGY OF TWINNING 



cutting out a section of worm from near the middle, 

 the anterior and posterior ends regenerate very frequently 

 into double or even triple heads or tails. Doubling or 

 twinning in the course of regeneration after cutting the 



body or appendages of 

 animals or plants is an 

 extremely common phe- 

 nomenon. I consider 

 that the slowness with 

 which early regrowth 

 occurs allows the axiate 

 organization of the tissue 

 to be lost, and new grow- 

 ing regions arise some- 

 times at two or more 

 places, thus resulting in 

 duplicity or triplicity. 



TWINNING IN THE MICRO- 

 DRILOUS OLIGOCHAETES 



The very recent ac- 

 count of Welch (192 1), 

 although not illustrated, 

 affords data of very con- 

 sider able significance 

 with reference to the 

 phenomenon which he 

 calls bifurcation. He 

 made a study of the 

 contents of over 500 co- 

 coons of Tubifex tubifex, 

 which were obtained 



Fig. 22. — ^Typical twin earthworm 

 of the species Allolohophora subriibi- 

 cunda, slightly bifid at the anterior 

 end and extensively bifid at the pos- 

 terior end. In other larvae the condi- 

 tion may be reversed or else one end 

 may be single and the other bifid. 

 (After Korschelt.) 



