1 68 THE BIOLOGY OF TWINS 



The point that interested me most in Sano's paper 

 has to do with the crown whirls of the two twin boys. 

 That of A is to the left of the median line and that of 

 B to the right. In detail the hair whorls of the two 

 are mirror-image duplicates. Such a condition is just 

 what we might expect in monozygotic twins, for there 

 is an intimate relation between hair arrangements, 

 scale arrangements, and friction-skin patterns; they 

 are all integumentary structures and are therefore 

 likely to exhibit mirror-imaging. Were there no other 

 evidence of the monozygotic character of these twins, 

 this condition of the hair whorls would go far to prove it.^ 



That many of the differences between such mono- 

 zygotic twins may be merely the result of differences 

 in developmental age is shown by an interesting pair of 

 very early human twins that were studied by Dr. F. E. 

 Chidester. One of these twins was about as advanced 

 as a month-old embryo and the other was in an early 

 primitive-streak stage. They both lay in a single 

 large amnion and were separated by a small area ol 

 extra-embryonic tissue. Dr. Chidester kindly sho'/,ed 

 me a drawing of a surface preparation of these twins, 

 but only a detailed study of sections will reveal tiie 

 interrelations of the two, and I shall be much interested 

 to learn the outcome of this study. 



Mental resemblances between duplicate human twins. — 

 If twins are strikingly alike structurally, it follows that 

 they must be alike functionally, since similar structures 

 could hardly have dissimilar functions. A pair of 



^I have just learned of an authentic case of reversal in a pair of 

 duplicate twins, one of whom is a colleague of mine here in the Uni- 

 versity of Chicago. One of these twn brethren is right-handed and 

 the other left-handed. 



