148 THE BIOLOGY OF TWINS 



3 places to the right of band 7. Note that these individ- 

 uals are not members of a pair, but are situated back to 

 back on opposite sides of the vesicle. 



Half-band reversals. — When in one fetus a double 

 scute occurs on, say, the right side near the margin and 

 in another fetus of the same set a similar element occurs 

 on the same half -band, but near the middle, there is said 

 to be a half-band reversal. Such conditions are exactly- 

 like the occasional reversals of pattern of the index fingers 

 of duplicate human twins (see Fig. 54), where the right 

 hands of the two show a reversal or mirror-image sjon- 

 metry in one finger. 



Set C 76 (see Fig. 49). Fetus IV has a double scute 

 twelve places to the left of the middle of band 4. Fetus 

 II has a similar element in band i twelve places from 

 the right margin. Note that if these two fetuses were 

 placed back to back (their normal position in the vesicle) 

 the left halves would be mirror-image duplicates in so 

 far as the double scute is concerned. 



Many more examples of extremely close resemblance 

 among the fetuses of a set might be cited, but for 

 further particulars the reader is referred to the complete 

 accounts pubHshed elsewhere. Suffice it to say that 

 the number of cases in which the double scute is unequiv- 

 ocally inherited is so large that, even when the location 

 of the element in mother and offspring and among the 

 fetuses of a set i"s quite diverse, we must still conclude 

 that the character is inherited, but lacks the localization 

 factor. , 



This leads to a brief discussion of the possible factorial 

 basis of scute- and band-doubhng inheritance. There 

 appear to be at least four unit factors involved: (i) a 



