INTRODUCTION 5 



inhibited, so that a failure of certain parts to divide 

 occurs and a single median structure appears. The 

 paired eyes, for example, may fail to develop and a 

 single median cyclopic eye may result. Just as there 

 may be an inhibition of the normal culmination of the 

 process of bilateral division, so there is frequently an 

 excess of division resulting in two bilateral structures 

 becoming completely separated, as when a single 

 individual develops two heads or two tails, while 

 the remainder is a more or less normal indiAddual. 

 The whole matter of bilateral development appears 

 to be quantitative in nature, in that the same 

 type of process may go not so far or farther than 

 normal. 



There are known for man as well as for the lower 

 vertebrates all stages of twinning, ranging from incom- 

 pletely bilateral forms that are subnormal, such as 

 cyclopic monsters, through various grades of super- 

 normal forms, such as two-headed types, double mon- 

 sters, and conjoined twins, culminating in completely 

 separate monozygotic or duplicate twins. That all 

 these types are merely phases of the same process of 

 bilateral doubling is, I believe, beyond question, as will 

 be shown in the chapters that foUow. 



The phenomenon of twirming then will be seen to 

 be a very fundamental process, one almost universal 

 in the field of biology. For wherever we have bilateral 

 doubling we have twinning in some form. 



All expressions of the twinning process involve the 

 same biological problems : those of s)Tnmetry, heredity, 

 and sex; but perhaps the process of duplicate twin- 

 ning proper, where two or more completely separate 



