94 THE PHYSIOLOGY OF TWINNING 



to bring about the physiological isolation of two points 

 somewhat separated from the median point — that point 

 most strongly inhibited — and the consequent production 

 of two potentially equivalent tail-buds. 



An interesting case of minimal twinning, similar to 

 that described for sea-urchin and starfish larvae in our 

 chapter on sjonmetry reversal, is described by Bateson 

 (1894) for a tadpole of Pelohates fuscus. Instead of 

 having one spiracle on the left side, as is normal for 

 Amphibia, there are paired spiracles of equal size. This 

 is cited by Bateson as a case of homoeosis. 



REPTILIA 



Partial twinning involving a more or less complete 

 doubling of anterior structures is probably fairly common 

 among reptiles; but relatively few workers have given 

 attention to the subject of reptihan embryology and 

 therefore few cases of twins or double monsters have been 

 reported. Reptilian development is so similar to that 

 of birds that this field of embryology has been relatively 

 neglected. That twinning does occur probably with 

 even greater frequency and with more success in the 

 reptiles than in the birds, I have many reasons for believ- 

 ing. While, as Bateson points out, there are no authen- 

 tic records of a double monster in mammals or in birds 

 having grown up in a wild state, there are many such 

 cases among the reptiles. Several of the older and some 

 of the newer, more critical writers have described 

 instances of complete or partial duplicity among the 

 snakes, involving mostly double or triple heads. At 

 least two authentic cases are on record. In some 

 instances the doubling involves only the most anterior 



