290 BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



end of metacarpal i was developed ; yet tho so-called trapezium is ab- 

 normally long and projects well down by the side of metacarpal ii. In 

 three cases where the pollex is developed in a rudimentary condition tho 

 trapezium is fused to metacarpal I. 



In other animals, such as the horse and ox, where there are well- 

 authenticated cases of vestigial polydactylism, the extra digits usually 

 represent the development of rudiments normally present in the embryo. 

 in the case of polydactyle swine, where the extra digits constantly make 

 their appearance in the region of digital reduction, it is but natural to 

 conclude that a rudiment of this digit, even though extremely vestigial, 

 is present in the embryonic manus. 



In cases where two (rarely three) extra digits are found in the poly- 

 dactyle manus, there are no modifications in the other parts. Moreover, 

 it is out of the question to consider digit i" as representing a prae- pollex 

 and 1* a pollex. Granting that the prae-poUex existed, there arc still 

 hisurmountable difficulties in the way of this interpretation. Both extra 

 digits develop on a single carpal element, the trapezium. They are sup- 

 plied by bifurcations of the same muscle tendon, innervated by the 

 divisions of the same nerve-branch, and may even be enclosed distally in 

 the same hoof. In addition, they are usually of the same size and sym- 

 metrical with each other. Thus their structure, and the fact that con- 

 ditions exist intermediate between a single undivided digit and two 

 completely separate ones, make it almost certain that the two extra 

 digits arise from the duplication of the pollex. 



Having found good evidence in favor of the vestigial origin of tho 

 extra digits, and that Gegenbaur's objections do not hold for all cases, 

 let us examine the evidence in favor of his theory that all cases of 

 polydactylism in the pig are due to duplication of the second digit. 



On examining the structure of two digits which are known to be dupli- 

 cations of a single one, we find that they are of nearly the same size, 

 symmetrical with each other, often enclosed in the same hoof, and borne 

 always on a single duplicated carpal element. They are supplied also 

 by duplications of the same muscle tendons, and innervated by the 

 bifurcations of the same nerve-branch. 



In the polydactyle cases which we have examined these are not tho 

 characteristic conditions. As we have seen, digits i and ii always differ 

 greatly in size, often in number of phalanges, and are not bilaterally 

 symmetrical. Digit i is never borne on the trapezoid, but on its own 

 proper carpal, the trapezium ; when the trapezium is apparently ab- 

 sent, it is really fused to metacarpal i, or to the trapezoid. The mus- 



