PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 299 



Rosenberg ('73) searched for sucli vestiges, but without success. 

 Ewart ('94), in tracing out the skeletal development of the limbs of the 

 horse, found cartiliigimnis nodules articulating iu an imperfect manner 

 with the distal epiphyses of metacarpals 11 and iv. The vestige 

 attached to digit 11 was the larger, and in some instances showed evidence 

 of division into two or three parts, which Ewart takes to be the funda- 

 ments of as many phalanges. 



Tins is an interesting and important discovery, since, if digit 11 is better 

 developed than iv in the normal embryo, we have a good explanation 

 for the fact that in polydactyle horses it is the second digit which is 

 of most frequent occurrence. Dissection of the manus of a foetus 35 cm. 

 long enabled me to confirm Ewart's work. There is thus conclusive 

 evidence that in the horse extra digits are frequently of vestigial origin. 

 The digital abnormalities of the Equidae can therefore be divided into 

 two distinct classes : 



(1) Vestigial cases, in which the extra digits are developed from 

 rudiments normally present in the manus of equine embryos and 

 extinct ancestors. 



(2) Teratological cases, which are malformations usually due to the 

 partial or complete duplication of the functional digit (m). 



VIII. Theories of Polydactylism. 



The occurrence of polydactylism has been attributed to two proximate 

 causes : (1) External influences, (2) Internal influences. 



1. External Influences. 



The supporters of this theory (Ahlfeld, '85-86, and Zander, '9l) would 

 explain all cases of digital variation as due to the pressure of amniotic 

 threads in ntero. This view accounts satisfactorily for the variation in 

 degree of digital duplications, but utterly fails to explain their fixed 

 position with reference to certain digits, and cannot apply to the 

 development of digital vestiges. Pressure from an amniotic thread 

 would naturally affect any finger or toe, whereas we know that poly- 

 dactylism in mammals is practically limited to the first or fifth digit, is 

 often bilaterally symmetrical in its occurrence, and may a0"ect both 

 manus and pes in the same individual. The abnormalities are also 

 strongly inherited, and the amniotic theory, if correct, would necessitate 

 admitting the inheritance of acquired characters. Although the duplica- 

 tion of organs has been artificially produced by Dareste ('91) and others, it 



