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  • Print publication year: 2010
  • Online publication date: July 2011

4 - Dichotomous branching

Summary

BREAKING SYMMETRY

I return to the topic previously mentioned at the beginning of Chapter 2. To my mind, the essence of biological development is symmetry-breaking. I have expressed this view earlier (Harrison 1981, 1993, Chap. 5) and in the latter reference pointed out an apparent conflict between this and a view expressed by Meinhardt (1982): ‘In most biological cases, pattern formation does not involve symmetry-breaking (although the proposed mechanism can perform this) … The asymmetric organism forms an asymmetric egg and the orientation of the developing organism is therefore predictable.’ I countered with:

Some of these [polarities] may be traceable back to the beginning of development (e.g. to the animal–vegetal polarity of an oocyte). This cannot, however, be the case for every polarity which is seen during development, for if all of these could be found at the outset, in proper spatial relationship, then, first, all development would be attributable to the inheritance of microscopic templates, and there would be no role left for the genome, and, second, the initial state would contain a miniature of the developed form. This would be equivalent to the long-discarded concept of the homunculus in the sperm.

There is no real conflict between these two views. In considering any developmental phenomenon that can be regarded as formation of a new pattern, one simply has to be careful about one's picture of the initial state and what features of it are changed or retained in the process under consideration.

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