NATURE, SCOPE, AND CAUSES OF TWINNING 3 



Twinning is essentially a process of regulation or 

 regeneration of a whole individual out of a prospective half 

 individual. All that is necessary in order to get two 

 individuals instead of one is to accomplish the division 

 of an embryo into two equivalent regions. Each region 

 is totipotent and soon reorganizes its own complete 

 axiate relations and becomes a whole individual. When 

 the separation of halves is incomplete, we get, in the case 

 of whole organisms, double monsters; and in the case of 

 special organs, merely double organs. 



THE DISTRIBUTION OF TWINNING THROUGHOUT THE 

 ANIMAL KINGDOM AND ITS SIGNIFICANCE 



In his book, Materials for the Study of Variation, 

 Bateson surveys the incidence of double monstrosity in 

 the animal kingdom. We find that it is found most com- 

 monly among vertebrates, not infrequently among anne- 

 lids and arthropods, occasionally in echinoderms of 

 various classes, rarely in coelenterates, cestodes, brachio- 

 pods, protozoans, I have never seen a reference to a case 

 of twins or double monstrosity in Mollusca, Nemertinea, 

 Nemathelminthes, Rotifera, Ctenophora, or Tunicata. 

 These latter groups are characterized by determinate 

 cleavage in its most deiim'te form. In such groups the 

 uncleaved Qgg is already highly organized, each part 

 of the Q^gg having a definite prospective value as an 

 organ-forming region. Hence, if the blastomeres are 

 isolated in two-, four-, or eight-cell stages they are not 

 able to produce whole individuals, but merely parts of 

 individuals. If divided in the two-cell stage, one cell 

 will develop into a left-half and the other cell into a 

 right-half embryo. It is no wonder then that in groups 



