REPRODUCTION 75 



chorions. They give every evidence of having 

 descended separately from two fertilized eggs. 

 They may be of the same or opposite sex. 

 After birth they resemble each other no more 

 than any other two children in the same fam- 

 ily. Identical twins, on the other hand, possess 

 only one chorion and are otherwise so related 

 to their foetal membranes as to give evidence 

 that they have descended from a single egg. 

 They are invariably of the same sex, and in 

 after life they are commonly very similar in 

 appearance. This similarity is often so marked 

 as to lead to their confusion even among their 

 near associates ; hence the ludicrous situations 

 furnished by the two Dromios and their mas- 

 ters in " Comedy of Errors ' ' as well as by their 

 classical progenitors, the Mensechmi. Identical 

 twins are never of opposite sex, as Viola and 

 Sebastian in "Twelfth Night," who, though 

 " born in an hour," must have been descend- 

 ants from separate eggs, if not the pure prod- 

 uct of a poet's imagination. The fact that iden- 

 tical twins are always of the same sex, and that 

 ordinary twins may or may not be, shows 

 that sex determination must be an operation 

 that occurs in the very early stages of devel- 

 opment and before the material ordinarily des- 



