220 



Heredity and Environment 



etc. (Figs. 75, 76). Or these double monsters may be produced 

 by division or budding of the embryo at a later stage of develop- 

 ment. In the human species, no less than in other animals, all 

 sorts of double monsters may be formed in this way by the par- 

 tial division of a single egg or embryo (Fig. 81). If the division 

 is slight the developed individual may show only the beginnings 

 of a division into two, as in two-headed forms ; if the division of 

 the egg or embryo is complete two separate and perfect individ- 

 uals may be formed from an originally single oosperm. When two 

 individuals are formed from a single egg they have exactly the 

 same heredity 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. 81, right end). On the other hand 

 twins which develop from different eggs do not have the same 

 heredity and may differ in sex as well as in other features ; they 

 are known as "fraternal" twins. 



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Fig. 81. 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.) 



