2 THE PHYSIOLOGY OF TWINNING 



Though thus limited, twinning is still a multifarious 

 process: there are many kinds of true twinning. If the 

 blastomeres of the two-cell stage of cleavage, destined 

 to develop into the right and left primordia of a single 

 individual, become physically or physiologically isolated 

 so as to develop independently of each other, two half- 

 sized, but otherwise normal, individuals result: these 

 are twins. If a young blastoderm becomes bilaterally 

 separated into two more or less equivalent blastoderms 

 and two independent individuals result, these are twins. 

 If a single blastoderm loses its axiate organization so 

 that two equivalent secondary points of gastrulation arise 

 instead of one, and two embryonic axes result, this is 

 twinning. If a single embryonic axis exhibits a tendency 

 for the two bilateral halves to grow independently and 

 a double-headed or double-tailed individual results, this, 

 no matter how complete or incomplete the isolation of 

 the two sides, is twinning. If an appendage such as a 

 hand or a foot becomes double instead of single, this is 

 also a phase of twinning. 



Twinning is a matter of great definiteness and depends 

 on the bilateral organization of the embryo. Rarely does 

 a division into more than two individuals occur, unless 

 one of the two redivides. Triplets are a hundred times 

 as rare as are twins. This very fact emphasizes the essen- 

 tial feature of twinning: its two-ness. Moreover, not 

 merely two individuals, but two equivalent individuals 

 are formed, which are nearly always symmetrically placed 

 with reference to each other. Neither one is the original 

 individual; neither one is secondary or subordinate to 

 the other; but both are equivalent individuals regener- 

 ated from a half-embryo. 



