174 THE PHYSIOLOGY OF TWINNING 



either one of them. If we were ever to find situs inversus 

 in twins separate except for the common yolk sac, it is 

 certain that the explanation of such an occurrence 

 would offer a problem of extreme difficulty. Normal 

 situs in both of these twins, which I have classed as 

 separate one-egg twins, is, then, only to be expected. 

 The same statement may be made for those double 

 monsters that are united only by lateral or ventral parts. 

 Such individuals may be interpreted as resulting from 

 secondary or mechanical fusions due to the close proxim- 

 ity of two embryonic shields. Unless double monsters 

 have a considerable single region which is like that of 

 a normal fish, I would say that such forms do not come 

 from the antimeric halves of a single axis and there- 

 fore would not be expected to show situs inversus. 



2. In the well-defined cases of twins with rather 

 extensive axial regions in common, such as the types 

 in which situs inversus occurs, our problem is not so 

 much to account for situs inversus (for this is the expected 

 condition in such bilaterally related twins) as to account 

 for the not infrequent occurrence of left-hand asymme- 

 try (situs solitus) in the right-hand component. This 

 seems to be the real symmetry reversal, for the right-hand 

 component is expected to be a mirror-image of the left, 

 and should show right-hand asymmetry {situs inversus) 

 of asymmetrical structures. The process is probably 

 one of regulation. The right-hand component, which 

 in some cases retains the symmetry of the half- 

 primordium from which it came, may become sufficiently 

 independent to develop its own asymmetry just as 

 though it were a single individual in no way influenced 

 by its twin component. Abundant instances were 



