VARIATION AND HEREDITY IN TWINS 139 



left side; in fetus III it is unilateral but on the side 

 opposite to that of the mother. Fetuses I and II have 

 in addition, in bands 4 and 5, respectively, a double 

 scute each. In fetus I the double scute is near the 

 right margin; in fetus II it is near the left margin. 

 This type of symmetry reversal is very common between 

 individuals of a pair and is called mirror-imaging. 

 Fetus I (belonging to one pair) has the more extensive 

 doubling on the left side, and the opposite fetus, III, 

 (belonging to another pair) has all the doubling on the 

 right side; this is an example of mirror-imaging involving 

 embryos derived from opposite sides of the egg. That 

 this is a genuine case of inheritance of doubling can 

 scarcely be doubted when it is considered that band 

 doubling occurs in only about 3 per cent of individuals 

 in the species. Its occurrence in mother and four off- 

 spring is more than a coincidence. 



Set K 30 (female fetuses) further illustrates the 

 mode of inheritance of scute and band doubling. A 

 somewhat simplified method of representing the 

 anomalies is here adopted (Fig. 43). 



In the mother there is in band i a minimal band 

 doubling involving two scutes located six places from 

 the left margin. In band 2 there is a scute doubling 

 ten places to the left of the middle. Some kind of 

 doubling appears in band i of all four fetuses. Fetus I 

 has a double scute in exactly the same spot where the 

 mother has an incipient double band; fetus II has 

 in the exact middle of the band a short band doubling 

 of three scutes and a double scute 14 places to the left 

 of the middle, or nearly in the place where the mother 

 has a double scute in band 2. In addition, fetus II 



