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 fourth, 
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 where 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, 
(phalanges of digits 11 and v in the horse). 
Bardeleben (785, ’85%, 86) answers Gegenbaur’s objections to re- 
versionary polydactylism in man, by advocating the prae-pollex 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 that 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 this 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 elasmobranchs. 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is not new, but was originally made by Cuvier, and he opposes the “ prae- 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 secondary 
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 
