INFLUENCE OF ENVIRONMENT 331 



sion into two, as in two-headed forms; if the 

 division of the egg or embryo is complete two 

 separate and perfect individuals may be 

 formed from an original single oosperm. 

 When two individuals are formed from a 

 single egg they have exactly the same he- 

 redity and accordingly they are always of the 

 same sex and are so similar in appearance that 

 they are known as "identical" or "duplicate" 

 twins (Fig. 76). On the other hand twins 

 which develop from different eggs do not have 



C I c II C II 



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Fig. 76. Diagram showing the different types of union of 

 double human monsters each being produced by a partial 

 division of a single egg or embryo. If the division is a 

 complete one, duplicate twins are formed, as shown by the 

 figures at the right end of each line. (From Wilder.) 



