VARIATION AND HEREDITY IN TWINS 141 



a band doubling involving seven scutes exactly in the 

 middle of band i. Nothing could show more clearly 

 the genetic equivalence of scute and band doubling, 

 for obviously the three doublings exactly in the middle 

 of band i i;n three fetuses (II, III, IV), one involving one, 

 another three, and another seven scutes, are genetically 

 equivalent and must be inherited from the condition 

 in the mother. It is quite common to find positional 

 reversals involving a shift from near the margin to the 

 middle. This type of symmetry reversal is scarcely 

 mirror-imaging, but is nevertheless of a kindred charac- 

 ter, doubtless due to similar factors. It will be noted 

 that fetus I has the same position of the doubling 

 as the mother has in the incipient band doubling, while 

 fetus II has inherited the double scute of band 2 of the 

 mother in its band i. Fetuses III and IV (a natural 

 pair) have a positional reversal of that of the mother, 

 III inheriting a scute doubling in the same place where 

 fetus IV inherits a band doubling. Again, it will be 

 seen that only the primary fetuses II and IV inherit 

 the band doubling, while the two secondary individuals 

 inherit the scute doubHng. Fetus II is the only one 

 that inherits both scute and band doubling and has the 

 doubHng involving two bands, as in the mother. 



Set K 4 (male fetuses) shows another set of con- 

 ditions (Fig. 44). The mother has in band i, beginning 

 seven places from the left margin, a small band doubhng 

 of three scutes. Fetus I has in band i, four places to the 

 left of the middle, a somewhat more extensive band 

 doubling involving five scutes. This is evidently a 

 case of inheritance with symmetry reversal of one side. 

 All the other fetuses have extensive band doubling. In 



