MERISTIC PHENOMENA 53 



some having their apices turned peripherally, some centrally, and 

 others in various oblique or transverse positions (Fig. 5). These 

 little leaves are thus comparable with seedlings, in that their 

 polarity is not related to, or consequent upon that of the parent 

 plant. They have in fact that "individuality," which we asso- 

 ciate with germinal reproduction. 



There are many curious phenomena seen in the behaviour of 

 parts normally repeated in bilateral symmetry which may some 

 day guide us towards an understanding of the mechanics of 

 division. A part like a hand, which needs the other hand to 

 complete its symmetry, cannot twin by mere division, yet by 

 proliferation and special modifications on the radial side of the 

 same limb, even a hand may be twinned. In the well known poly- 

 dactyle cats a change of this kind is very common and indeed 

 almost the rule. When extra digits appear at the inner (tibial) 

 side of the limb, they are shaped as digits of the other side, and 

 even the normal digit II (index) is usually converted into the 

 mirror-image of its normal self. The limb then develops a 

 new symmetry in itself. Nevertheless it is not easy to interpret 

 these facts as meaning that there has been some interruption 

 in the control which one side of the body exercises over the 

 other. The heredity of polydactylism is complex but there is 

 little doubt that the condition familiar in the Cat is a dominant. 

 In some human cases also the descent is that of a dominant, but 

 irregularities are so frequent that no general rule can yet be 

 perceived. The dominance of such a condition is an exception 

 to the principle that the less-divided is usually dominant to the 

 more-divided, a fact which probably should be interpreted as 

 meaning that divisions are of more than one kind. 



Among ordinary somatic divisions, whether of organs, cells, 

 or patterns of differentiation, the control of symmetry is usually 

 manifested. There is however one class of somatic differenti- 

 ations which are exceptionally interesting from the fact that they 

 may show a complete independence of such geometrical control. 

 The most familiar examples of these geometrically uncontrolled 

 Variations are to be seen in bud-sports. The normal differ- 

 entiation of the organs of a plant is arranged on a definite geo- 



