94 ROSS G. HARRISON 
II. The nearer of the two extra appendages is in structure and posi- 
tion formed as the image of the normal appendage in a plane mirror 
placed between the normal appendage and the nearer one, at right 
angles to the plane of the three axes; and the remoter appendage is 
the image of the nearer in a plane mirror similarly placed between the 
two extra appendages. 
Transverse sections of the three appendages taken at homologous 
points are thus images of each other in parallel mirrors. 
In the vertebrates Bateson marshals a large amount of mate- 
rial, of which about fifty cases are in amphibians. i''^' At the tune 
Bateson's book was written, however, little or nothing was known 
regarding the origin of supernumera y appendages in either the 
arthropods or the vertebrates. Since then a large amount of 
experimental evidence has accumulated to show that they may 
be formed by super regeneration, especially by regeneration from 
complex or irregular wound surfaces. ^"^ The evidence all cor- 
roborates Bateson's main gene, alizat ion regarding the relation of 
symmetry of supernumerary limbs, and there are practically no 
exceptions. ^°^ 
The miportance of double supernumeraries (Bruchdreifachbil- 
dung, la doppia rigenerazione inversa, see p. 95) is emphasized 
in the papers by Emmel ('07) and Delia Valle ('13), and this con- 
ception is given prominence in Przibram's more general discus- 
sion of the question ('09, p. 234). 
^"^ See Bateson (pp. 554-5) for a discussion of the older literature. 
1°^ In the amphibians the investigations of Barfurth ('94), Giard ('95), Tornier 
('97, '00, '05, '06), Lissitzky ('10), Fritsch ('11), Kurz ('12), Delia Valle ('13), 
and others have added much to our knowledge of the subject. In the crustaceans 
Przibram ('02), Reed ('04), Zeleny ('05), and Emmel ('07) have reported experi- 
ments which, though not so numerous, are none the less important. The more 
recent literature is fully discussed in many of these papers, especialh' in those 
of Lissitzky, Fritsch, and Delia Valle, to which the reader is referred for details. 
105 ^ remarkable exceptional case has recently been described by Dawson 
('20) in a lobster, in which there is an extra pair of chelipeds attached to the 
normal. The two extra chelae are mirror images of one another, but the one 
nearer the primary claw is not mirrored from the latter, but is of the same side. 
Furthermore, the primary claw is a 'nipper,' while the supernumeraries are both 
of the 'crusher' type, so that the case proves to be likewise an exception to Przi- 
bram's rule ('11), according to which, in heterochelous forms, the extra appen- 
dages are of the same type as the primary. The case described by Cole ('10), 
also in a lobster, is an almost diagrammatic example of Bateson's rule, if allow- 
ance is made for the effects of torsion. 
