TWINS AND HEREDITY 



477 



type of twin in which brother and sister are so much alike that one 

 may take the place of the other. Such twins are purely literary sig- 

 nificance and justified only for dramatic and fictional purposes. 



Siamese twins are of the same origin as duplicate twins. They 

 are the product of the division of a single individual into two, but in 

 their case the division has been incomplete. Sometimes they are 

 double in the head region and single back of that; sometimes they 

 are double in the lower trunk region and single anterior to that; and 



Fig. 90. — A pair of typical fraternal twins, showing differences in eye color, 

 features, body build, etc. 



sometimes they are double in the head and leg regions and united 

 only in a small area at the hips or sides. Cases of Siamese twins that 

 survive are exceedingly rare, the number recorded being so small that 

 they could be counted on one's fingers. Yet it has been my privilege 

 to have examined within recent years two of these cases. 



Fraternal twins are quite different in origin. They arise from the 

 simultaneous fertilization of two eggs and, except that they develop 

 together in the same uterus, they have no closer kinship than ordinary 

 brothers and sisters. They may be of the same sex or of opposite 

 sexes in just the same way that families of two children may be both 

 boys, both girls, or a boy and a girl. Fraternal twins are therefore, 



