4 THE PHYSIOLOGY OF TWINNING 



with strictly determinate cleavage we find no examples 

 of twinning, for twinning requires a totipotency of blas- 

 tomeres or regions of the blastoderm. 



Equally significant is the fact that twinning is 

 especially characteristic of the vertebrates, a group in 

 which cleavage is most clearly indeterminate ^ in the sense 

 that the early cleavage cells appear to be totipotent, i.e., 

 each able, if isolated, to produce a whole individual. 

 In the vertebrates, although the axes of polarity and 

 symmetry are already laid down in the undivided egg 

 and the axes of the embryo tend to coincide with those 

 of the egg, it appears to be relatively easy to break down 

 the axiate relations either in the egg or in various embry- 

 onic stages, and then to estabhsh new axes. This all 

 means that, apart from their axiate integration, the 

 blastomeres, until a relatively late period of cleavage, 

 are potentially equivalent, no blastomere containing any 

 unique organ-forming material, but each being essentially 

 germinal in character and able under the proper condi- 

 tions to become a new apical point and to establish new 

 axes of polarity and symmetry. This extreme versatility 

 of the elements of the vertebrate blastoderm is, in my 

 opinion, the secret of its twinning capacity. There is 

 a good reason then why the vertebrates are par excellence 

 the favored group for twins. 



The other group in which twinning occurs readily 

 is that of the echinoderms, in which cleavage is also 

 decidedly indeterminate. In the annelids and arthro- 

 pods, where partial twinning occurs only occasionally or 

 is confined to a few types, we find that cleavage is strictly 

 intermediate between the extreme determinate and the 

 extreme indeterminate types. Thus again the parallel- 



