176 THE BIOLOGY OF TWINS 



hereditarily, controlled in the same way on both sides of 

 the body of those individuals who are bilaterally asym- 

 metrical. 



The unilateral appearance of an inherited unit 

 character, such as a friction-skin pattern, almost 

 certainly implies some unilateraUty in the somatic 

 distribution of the differentiating factor for this charac- 

 ter. Whether the character appears in one or in both 

 of a pair of twins (which are genetically equivalent to 

 the right and left sides of a single individual) , or, finally, 

 whether it appears in one, two, three, or four members 

 of a set of armadillo quadruplets, depends on whether 

 the differentiating factor is distributed during the 

 earliest cleavage in a unilateral or bilateral fashion; in 

 other words, whether, with respect to the differen- 

 tiating factor in question, the earliest cleavages have 

 been equational or differential. All of the cells de- 

 rived from the blastomere that receives the factor 

 will produce individuals with the character, unless, as 

 often happens, subsequent cleavages still further limit 

 the distribution of the factor by repeated differential 

 division. 



Thus we appear to have the possibility of a segrega- 

 tive mechanism, which, in so far as an individual or 

 set of monozygotic twins is concerned, might give 

 results that would resemble the segregation of unit 

 characters in the maturation division of the germ-cells. 

 That segregation of unit characters resulting in the so- 

 called purity of gametes probably has its counterpart 

 in the segregations that occur in the early cleavages 

 in the armadillo and is not confined to the gonads 

 seems certain; there appears to be a parallel segregation 



