DOUBLE TAILS IN VERTEBRATES 191 



other abnormalities such as cyclopia, humpback, abnor- 

 mal heart. It seems only reasonable, then, to infer 

 that the same types of causes are responsible for the 

 excessive growth seen in double tail as are responsible 

 for the defects above named. In all cases it is very 

 plain that the rate of development from an early period 

 has been decidedly retarded as compared with that 

 seen in the corresponding pure-bred embryos. 



DOUBLE-TAILED GOLDFISH 



One species of fish is characteristically double tailed 

 even in nature — the goldfish {Cyprinus auratus). The 

 production of the double-tailed conditions is, like that 

 of other morphological oddities in these fishes, under the 

 control of the breeders, who are experts in these matters. 

 Two particular kinds of double tails are common: (a) 

 those in which each half of the double tail is essentially 

 a complete tail and the two tails lie side by side, only 

 united dorsally at the point of their union with the body; 

 {h) those in which the two tails are more or less com- 

 pletely fused by their dorsal margins in such a way that 

 the double fin is a three-lobed affair. All stages of com- 

 plete and incomplete doubling occur in any lot of fishes 

 derived from one batch of eggs. Sometimes the doubling 

 involves the vertebral column and sometimes only the 

 fin rays or vertebral arches. 



The condition of double tail does not seem to be 

 definitely heritable in goldfishes. Normal parents pro- 

 duce many offspring with double tails and double-tailed 

 individuals produce many normals. There is inherited, 

 it seems, merely a high degree of susceptibility to the 

 conditions responsible for doubling. What these are we 

 shall now inquire. 



