192 THE PHYSIOLOGY OF TWINNING 



THE CAUSES OF DOUBLE TAILS 



Tail doubling is in my opinion a phenomenon very 

 similar to head doubling, but is probably characteristic 

 of a later developmental period. 



Thanks to the recent experiments of Dr. Hyman 

 (192 1) we now know that the fish embryo, during a 

 relatively early germ-ring stage, forms the rudiments of 

 a tail-bud, the equivalent of the Knopf of Kopsch. This 

 posterior region of the primary axis has from an early 

 time a very high relative susceptibility to inhibiting 

 agents, such as anaesthetics, lack of oxygen, cold. Even 

 in presomite stages of the embryo there is present a 

 double gradient, similar to that in annelid worms, with 

 points of high activity and susceptibility at the anterior 

 and at the posterior ends and with gradients of suscepti- 

 bility running both backward and forward. After the 

 anterior parts of the axis are fully established and have 

 undergone considerable differentiation, the posterior end 

 of the axis remains an actively growing region. It is 

 evident that after a period of retardation this actively 

 growing tail-bud region undergoes bilateral fission in 

 order to form double tails. Abundant evidence is at 

 hand indicating that the twinning process of the tail is 

 due primarily to a slowing- down of the developmental 

 rate so that the two bilateral halves lose their co- 

 ordination and proceed independently, each regulating 

 for itself a bilateral symmetry more or less complete. 

 Mirror-imaging is under these conditions quite the 

 expected thing, and the expectation is always realized. 

 Thus we see that twinning of the tail is extremely like 

 twinning of the head and body, and doubtless depends 

 on the same factors operating at a different time. 



