68 THE PHYSIOLOGY OF TWINNING 



the other, be more severely inhibited and hence develop 

 less normally than the other. The question arises as to 

 whether we have any evidence that such a physiological 

 as)anmetry of the bilaterally equivalent primordia exists. 

 There is in abundance exactly such evidence. Anyone 

 who has engaged in the experimental production of 

 monsters in fishes cannot help but recall that one of the 

 commonest types of deformity is unilateral in its occur- 

 rence. One finds many embryos with a normal and a 

 subnormal eye, with one pectoral fin smaller than the 

 other. Another very common type of deformity is 

 that in which one whole side has been relatively sup- 

 pressed so as to cause the animal to have a curved or 

 spiral axis. Now, if in a single untwinned individual 

 one side may be inhibited while the other remains 

 normal, there is no difficulty about explaining the 

 difference between the bilaterally placed components of 

 double monsters. When once one component becomes 

 relatively inhibited it might be secondarily further 

 suppressed by the stronger twin through the medium 

 of the circulation and might ultimately be almost or 

 quite obliterated. Several instances have been cited, 

 both in human twins and in fish twins, in which such a 

 relatively inhibited component of a true double monster 

 is seen to be reduced to the condition approximating 

 that of a parasite on the body of the normal component. 



THE FISSION THEORY OF THE ORIGIN OF 

 CONJOINED TWINS IN FISHES 



This theory depends on the conception that a bilateral 

 organism is in a sense a dual individual. A limited 

 amoimt of concrescence obviously occurs even in forms 



