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 thun^b. 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 hexadactylo 

 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 vi 

 utero by the pressure of amniotic threads. 



This explanation was first proposed by Ahlfeld ('85-86), wlio 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 bo met 

 with in cases of polydactylism and polymelia ; for, the earlier the amnion 

 presses upon an extremity of tlie embryo, the more complete and far- 

 reaching will be the duplication produced. 



Marsh ('92), in treating of polydactylism inthe horse, gives little weight 

 to the fact that the ungual phalanges of tlio supernumerary digits never 

 revert to the partially cleft condition peculiar to the fossil horse. But 

 he concludes (p. 351) tliat "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- 



