547 
sesamoid bone, but as the remnant of this post-minimal digit, while 
the occasional sesamoid on the trapezium to which a slip of the ex- 
tensor ossis metacarpi pollicis is attached, may be a pre-hallux (Keırn). 
The two cases above described show no connection with the trapezium 
and appear to be due to splitting of the terminal or distal part only 
and so probably are not true pre-halluces. This theory is very attrac- 
tive but it is entirely hypothetical and most unlikely that any ancestor 
of man possessed seven digits. 
2) The fission theory namely, that there has been a splitting of 
one of the primitive digits does not lead us to the explanation but 
merely states the fact and leads one to the next theory. 
3) The Amniotic band theory where it may be supposed that 
there had been some abnormal band in the amnion as often happens 
and this has, at an early period, for a shorter or longer time, as the 
case may be, pressed on the digital buds and thus caused fission. 
The relative amount of, and length of time which the pressure lasts, 
will so determine the extent of the splitting. 
The two cases above described are most easily explained by the 
Amniotic band theory as here we have no distinct history of heredity 
whereas those cases, where heredity is a prominent factor must be 
termed atavistic and explained under the evolution theory. 
Thus we have two clear classes of Polydactyly which simulate 
each other very closely. 
1) The atavistic class with a distinct history of heredity usually 
showing symmetrical characteristics. 
2) The class where we get fission by artificial means which is 
asymmetrical and has no history of heredity and where the condition 
is termed Schistodactyly. 
References. 
Keırn, Human Embryology and Morphology. 
Pryor, Med. Record, Vol. 19, No. 18. 
Pick, Lancet, 1907, Vol 2, No. 20. 
ATTLEE, Ibid. 1907, July 20. 
Wirurow, Ibid., 1907, Aug. 24. 
Green, Ibid., 1907, Sept. 21. 
35* 
