CHAPTER I 



THE NATURE, SCOPE, AND CAUSES OF TWINNING 



THE NATURE OF TWINNING 



Strictly speaking, twinning is twaining or two-ing — 

 the division of an individual or an organ into two equiva- 

 lent and more or less completely separate individuals 

 or organs. The term dichotomy is almost a perfect 

 synonym for twinning, for it means literally a process of 

 division into two parts. When, as in the specific twinning 

 of the armadillos and in the sporadic twinning of other 

 species, more than two offspring are produced from one 

 egg, the condition may be still called twinning because 

 the process of dichotomy is simply repeated two or more 

 times. Thus we may have single twinning, double, 

 triple, or even quadruple twinning. The most advanced 

 phase of twinning is that seen in the South American 

 armadillo Dasypus hyhridus, which undergoes always as 

 many as three twinning processes and often proceeds well 

 into a fourth, so as to produce up to twelve embryos 

 from a single egg. I do not consider polyembryony as 

 exhibited among the parasitic hymenoptera an instance 

 of true twinning: this, for reasons that wiU be made 

 clear later. Furthermore, since twinning is essentially 

 a division of one into two, we are not justified in retaining 

 within the category of twins any cases in which two 

 individuals have originated from two germ cells originally 

 separate. In this narrower sense, therefore, twinm'ng 

 proper is always monozygotic or one-egg twinning. 



