262 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



cases. Of these, there was only one instance \vhere the supernumerary 

 digits occurred on the posterior extremity. In four cases the abuor- 

 mahty was found on botii fore feet ; and in all the specimens which he 

 himself examined, or which were described by other observers, the extra 

 digits occurred on the radial, or thumb, side of the manus. The ab- 

 normalities as figured by Ei'colani (Tav. 1, Fig. 1-G) consist in the 

 presence of from one to three supernumerary digits. He found also 

 that the trapezium of the carpus was well developed in most cases, and 

 occasionally duplicated. In two cases, however, it was entirely absent, 

 and Ercolani tlierefore concludes that its presence in connection with 

 the supernumerary digits is no proof that polydactylism is atavistic ; 

 for the trapezium is present also in most normal swine. Its absence is 

 a deformity by defect and may occur in the normal manus. 



Blanc ('93) considers most of the cases of polydactylism in swine as 

 due to reversion. He figures four types : (1) Manus with an extra digit 

 of two phalanges, representing the developed pollcx (Fig. 7, p. 70). 



(2) An extra digit of three phalanges, which he regards as the pollex 

 strongly developed ; digit ii is also abnormally large (Fig. 8). (3) 

 ]\ranus resembling (2), but with a small digit of two phalanges and a 

 rudimentary metacarpal occurring on the radial side of digit i (Fig. 9). 

 (4) j\fanus of six completely formed digits, the two supernumerary 

 being large and of nearly equal size (Fig. 10). Blanc considers types 



(3) and (4) as reversions to the hexadactyle ancestor of mammals. 

 Two other cases are figured to illustrate the duplication of digits 

 I and II. 



Gegenbaur ('80) examined two cases of polydactylism in the manus 

 of the pig. In one specimen the carpals had been entirely removed, in 

 the other they Avere partly cut away. From this fragmentary material 

 he draws his conclusion, — that all cases of polydactylism in swine are 

 monstrosities and not due to atavism. The conclusions of Blanc and 

 Gegenbaur are thus completely contradictory. 



If we reject the prae-pollex theory as untenable, the hexadactyle cases 

 regarded by Blanc as reversions must be accounted for in some other 

 way. On the other hand, (iegenbaur bases his arguments on the slender 

 evidence of two mutilated specimens; there is need therefore of further 

 investigation into the structural conditions peculiar to polydactyle swine, 

 before his refutation of reversion can be accepted. In proceeding with 

 our description of digital abnormalities in the pig we shall keep especially 

 in mind their bearing on this question. 



