158 THE PHYSIOLOGY OF TWINNING 



in size and degree of normality of the components of 

 fish double monsters. In conclusion I would also like 

 further to urge another explanation of the inequality of 

 the two components that was previously offered in the 

 case of fishes (see pp. 67, 68) : that the two bilateral pri- 

 mordia, after physiological isolation, are somewhat inde- 

 pendent and are not affected to the same extent by the 

 prevailing growth-retarding agencies that have been 

 responsible for twinning. ' There is just as much reason 

 for the two bilateral components of a pair of conjoined 

 twins to be different in their susceptibility to inhibiting 

 agents as there is for the two sides of a single individual 

 to be differently affected. The same causes are almost 

 certainly concerned in the same ways in both cases. 



