The fact that ncarly all humans have their heart on the left hand side is taken for granted by most people, without appreciating the complexity of the explanations needed to account for why this is consistently so. In many organisms we can understand, in principle, how the antero-posterior, and dorw-ventral, axes are established, but we have no straightforward concept to explain how the left side of an early embryo comes to be different from its right side, even though one appears to be the exact mirror image of the other, at an early stage. The purpose of this very timely Ciba Symposium is to summarize what i s known of this extraordinarily difficult biological problem.
This volume contains 16 contributions ranging from discussions of asymmetry in molecules to the prevalence of left or right handedness in various human conditions. As is customary in Ciba Symposia, the extensive discussions after each paper are printed. This i s probably the only published survey of current theories and facts on the problem of left-right asymmetry. and therefore stands apart from those reviews or symposia which appear every few months on more fashionable subjects. This is therefore an exceptionally useful and worthwhile production.
To me, three fundamental points stand out in this field. First the great majority of examples of left-right asymmetry cannot be explained by a maternal influence, either genetic or behavioural. Second, there are some remarkable cascs, such as situs inversus in the mouse, in which a single locus drastically alters the leftright asymmetry of the embryo. The remarkable observation about such cases is that the mutation only randomises the natural bias, but never causes a complete switch of asymmetry from, for example, left to right handedness. The third striking point to emerge, from thc chapter by Chothia, is that it is not currently possible to conceive of any easy route by which asymmetry at the molecular level, e.g. of an L-amino acid, can be coiiverted into asymmetry at the gross morphological level.
My general impression of the field is that it is oversupplied with theories, most of which are neither tested nor obviously testable, and a large amount of what has been written is descriptive. This may well be' an acceptable state of affairs for a new field. Notable exceptions are the articles by Wood (nematodes) and Jost (Amphibia) in which a range of experimental approaches are reviewed. The best prospects of thc field seems to be the mapping, and eventual characterization, of genes such as situs inversus, and the article by