Summary

This essay on the symmetry of reduplicated lateral appendages starts from an explanatory principle, viz., that any step of ontogenetic differentiation which reduces the symmetry of an organ (e.g., from radial to bilateral symmetry, or from bilateral symmetry to asymmetry) requires additional orienting information. From this principle it is argued that a normally asymmetrical lateral appendage, lacking some necessary piece of orienting information, will only be able to achieve bilateral symmetry, i.e., instead of a normal asymmetrical appendage, the result will be a bilaterally symmetrical doublet.

To examine this explanatory principle, the writer has at-tempted to construct a hypothesis to explain Bateson's Rule as this regularity is exemplified in the rare supernumerary double legs of Coleoptera. In the construction of this hypothesis, it was assumed that morphogenetic orienting information may undergo transformation from one type of coding to another, and that each transform or code is subject to characteristic limitations:

(a) The information may be embodied in gradients (perhaps biochemical). In this coding, the information can be diffused from neighboring tissues and provide the first determinants of asymmetry in the developing appendage. It is suggested that information coded in this way is only briefly available, and that once the asymmetry of the limb is established, the information continues to exist, but trans-formed into morphology.

161 Bateson (Materials ..., op. cit., p. 507) describes and figures one doubtful exception to this statement. This is a reduplication in the left hind tarsus of platycerus caraboides.

162 G. Bateson, "Minimal Requirements for a Theory of Schizophrenia," A.M.A. Archives of General Psychiatry, 1960, 2: 477-91.

(b) It is suggested that information coded as morphological difference is essentially static. It cannot be diffused to neighboring tissues and it cannot inhibit branching. It can, however, be used by a branch which at its inception shares tissue with the primary limb from which it branches off. In this case, the information passed on by the method of shared periphery will be necessarily inverted: if the primary be a right, the branch will be a left.

(c) The information in morphological form being (by hypothesis) unable to inhibit branching, the asymmetry of a growing primary must be preserved by a centro-peripheral gradient—not itself a determinant of that asymmetry.

(d) It is suggested that the loss of such a centro-peripheral gradient might have two effects: that of permitting branching and that of depriving the resulting branch of one dimension of necessary orienting information; so that the branch can only be a bilaterally symmetrical unit with a plane of symmetry at right angles to the lost centroperipheral gradient.

The data on reduplication in the experimentally trans-planted limb buds of amphibia are also examined. It is argued that these data are not to be explained by simple loss of orienting information. Simple loss, it is suggested, will expectably result in equal and synchronous bilateral symmetry. The amphibian reduplicates are, in general, unequal and successive. In a few cases, synchronous and equal reduplication occurs in the amphibian experiments, especially in heterotopic implants. Such cases could perhaps be regarded as due to simple loss of orienting information.

5.4.6 Postscript, 1971

Compare the bilateral symmetry in the supernumerary doublet of the beetle's leg with the bilateral symmetry in the sweet pea or orchid flower. Both in the plant and in the animal, the bilaterally symmetrical unit comes off from a point of branching.

In the plant, the morphology of the fork provides information enabling the flower to be not radially but bilaterally symmetrical, i.e., information which will differentiate the "dorsal" standard from the ventral lip of the flower.

In the doublet on the beetle's leg, the plane of bilateral symmetry is orthogonal to that in the flower.

We might say that the information which the beetle's leg has lost is precisely that information which the plant creates by the act of branching.

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