PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 247 



digits are produced by duplication or intercalation. He regards all 

 cases of polydactylism in the pig as due to the splitting of one of the 

 functional digits, and holds therefore that they are monstrosities. 

 Polydactylism in the horse, he admits, may be atavistic, as (1) the 

 reversion is to a closely related ancestor ; (2) in Hipparion, a three- 

 toed fossil horse, the second digit is better developed than the fourtli, 

 and in polydactyle horses the second digit is the one which most usually 

 appears ; (3) the rudiments of the extra digits may be present in the 

 embryo. Atavism Gegenbaur divides into two types: (1) Palaeo- 

 genetic, or cases wliere the fundament of an organ is always present in 

 the embryo, and may develop, or may degenerate (centrale of man) ; 

 (2) Neogenetic, or cases where the organ is absent even in the embryo, 

 (plialanges of digits ii and v in the horse). 



Bardeleben ('85, '85% '86) answers Gegenbaur's objections to re- 

 versionary polydactylism in man, by advocating the prae-poUex theory. 

 He maintains that the cartilaginous elements found on the radial side 

 of the hand and the tibial side of the foot are rudiments of a " prae- 

 pollex " and " prae-hallux," respectively, and not sesamoids, as had been 

 previously maintained. Also tliat the pisiform of the carpus and the 

 tuberositas calcanei of the tarsus represent the rudiments of '' post- 

 minimi." The manus and pes of primitive mammals were therefore in 

 his opinion heptadactyle, and polydactylism in man and other mammals 

 is simply reversion to tins ancestral seven-toed condition. 



Boas ('85, '90) considers polydactylism in the horse and ox as due to 

 reversion. The extra digits formed do not represent simply the per- 

 sistence of an embryonic condition, for in the polydactyle ox phalanges 

 are formed in the extra digits, and these elements are normally absent 

 in the embryo. 



Albrecht ('86) points out that in man the greater number of poly- 

 dactyle cases consist in the duplication of a single digit. This he as- 

 sumes to be reversion to the bifid fin-rays of the elasmol)ranchs. He 

 distinguishes this type of polydactylism (false hyperdactyly) from that 

 found in animals where the number of digits is less than five (true 

 hyperdactyly). Albrecht is supported in his view by Kollman ('88). 



Gegenbaur ('88) states that the discovery of the so-called " prae-pollex " 

 is not new, but was originally made by Cuvier, and he opposes the "prae- 

 pollex " theory of Bardeleben on the following grounds : (1) these doubtful 

 rudiments never form true fingers, and their development is secondaiy 

 to that of the other digital bones ; (2) polydactylism in man cannot 

 be explained by it, for supernumerary digits occur on the ulnar as well 



