248 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
as on the radial side of the carpus, and they may also be interpolated 
between the other digits; (3) when the “ prae-pollex” is present, no 
correlated changes have been observed in the carpus and other parts of 
the manus; (4) its inheritability is no proof of reversion to a palin- 
genetic digit, for all monstrosities are inherited. Bardeleben’s theory is 
therefore an “ unbegriindete Behauptung,” and polydactylism in man is’ 
due to doubling of the normal digits. 
Zander (’91) describes in some detail a case of hexadactylism in man, 
concluding that the abnormality was produced by the splitting or dupli- 
cation of the fundament of the normal thuwb. He discusses at some 
length the different theories which have been advanced to account for 
polydactylism. Reversion and the assumption of Bardeleben he rejects 
on the following grounds: (1) the rudiments of the prae-pollex are of 
secondary formation, and therefore are sesamoids, not digital vestiges ; 
(2) Kiikenthal (’89-93) has shown that the sixth digit found in Delphi- 
nus leucas is produced by the splitting of the fifth digit in the embryo ; 
(3) the most primitive fossil reptiles, the Ichthyopterygia, possessed, 
according to Baur (’87), only five digits, and therefore the hexadactyle 
condition must have been brought about later, either by duplication 
of the primary digits, or by neomorphic development on the ulnar side of 
the extremity ; (4) no case has been observed where the “rudiments” of 
Bardeleben have developed into supernumerary digits. On the contrary, 
the extra fingers of man are usually attached distally, where no rudi- 
ments exist. Polydactylism in man, therefore, cannot be atavistic, but 
is due to duplication of normal digits. This duplication is caused im 
utero by the pressure of amniotic threads. 
This explanation was first proposed by Ahlfeld (85-86), who observed 
at the birth of an infant with a divided thumb that an amniotic thread 
was still present in the fissure of the duplicated digit. This theory 
accounts most satisfactorily for the different stages of division to be met 
with in cases of polydactylism and polymelia ; for, the earlier the amnion 
presses upon an extremity of the embryo, the more complete and far- 
reaching will be the duplication produced. 
Marsh (92), in treating of polydactylism in the horse, gives little weight 
to the fact that the ungual phalanges of the supernumerary digits never 
revert to the partially cleft condition peculiar to the fossil horse. But 
he concludes (p. 351) that “ All the examples of polydactylism in the 
horse which the writer has had opportunity to examine critically are 
best explained by atavism, and many of them admit of no other ex- 
planation. Taken together with their great frequency they clearly indi- 
