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CHARLES R. STOCKARD 



individuals. In other words, monophthalmia, cyclopia, anoph- 

 thalmia, deformed brains, and branchial structures occur among 

 single specimens developing along with the double ones. 



We may now consider the condition of the smaller component 

 in double specimens in which the components are two complete 

 individuals, or conjoined twins. . 



Figs. 18 and 19 Two views of a rare double specimen in which the compo- 

 nents differ slightly in size yet both components are deformed. The left larger 

 head has only one eye, the left; it is otherwise perfect, as the figures show. The 

 right smaller head is completely eyeless and grossly deformed in the anterior 

 portion. In this case the larger left head is by chance defective just as any 

 single individual might be. 



Figure 20 illustrates normal equal-sized identical twins at- 

 tached to a common yolk-sac. The development of a teleost 

 embryo on a large yolk-sphere and the structure of its yolk-sac 

 prohibits a free separation of identical twins and they always 

 remain joined as shown in this figure. 



