i62 THE PHYSIOLOGY OF TWINNING 



right- and left-hand homologous organs in a bilateral organism 

 is essentially a twinning process." This author also observes that 

 "the whole matter of bilateral development appears to be quanti- 

 tative in nature, in that the same type of process may go not so 

 far or farther than normal." Just as there may be an inhibition 

 of the normal culmination of the process of bilateral division 

 (as in the median cyclopic eye), so there is frequently an excess 

 of division resulting in two bilateral structures becoming com- 

 pletely segregated, as when a single individual develops two heads 

 or two tails, while the remainder of the organism is a more or 

 less normal individual. Newman, like Bateson, regards the phe- 

 nomenon of twinning as a fundamental process which is almost 

 universal in the field of biology. 



From this point of view, hemihypertrophy may be interpreted 

 as an atypical or incomplete form of twinning, a variant of the 

 same process which may produce a double-headed monster or a 

 perfectly ordinary normal individual — an ordinary individual 

 being an organism in whom there has been a precisely balanced 

 inhibition on the biologic process of bilateral doubling. 



DISCUSSION 



If we accept hemihypertrophy as a mild form of 

 unilateral gigantism we must view the hypertrophied 

 side as abnormal and the small side as relatively normal. 

 From this point of view we are led to compare the 

 condition in question with one that has been frequently 

 noted in connection with various unilateral monstrosities 

 in fishes which are the result of certain growth-retarding 

 agents. It very commonly happens in such fish embryos 

 or larvae that the organs of one side of the body develop 

 much more rapidly than those of the other side and we 

 get curved or even spiral forms. Frequently the eye 

 of one side is much more defective than that of the other, 

 and the same is true of fins and other paired structures. 

 In view of these facts we might interpret these cases, 



