230 



ESSENTIALS OP ZOOLOGY 



two of which showed these complete characteristics on all feet, one 

 of the litter had only an extra toe on one foot, and the fourth was 

 normal. Both hind feet of this animal had five well-developed 

 digits instead of the usual four. This condition is a hereditary one 

 and is brought about by partial duplication of elementary structures. 



Conjoined Twins 



Instances occur in which individuals of about equal size or of 

 unequal size are fused together. If this occurs at the hips with 

 dorsal sides together, the condition is usually known as Siamese 



'=»f__,,'.*^r- 



^% 



4^ 



Fig. 70. — Conjoined twins of cat showing single head, but double trunk and ap- 

 pendages. 



twins. There are a number of different varieties of fusion, includ- 

 ing the head region, chest region, or complete fusion of trunk into 

 a single body with two heads and vice versa. A chicken with two 



