Preferential diffusion along dislocations may give rise to several limiting types of behaviour, in some of which the kinetics of diffusion remain, on a macroscopic scale, in apparent agreement with Fick's law. The various possible types of behaviour are classified, and the conditions required for each type are discussed in detail, with complete derivations where necessary. The features of each type of diffusion which will appear in different experimental techniques are discussed.Self-diffusion in the alkali halides is discussed in the light of this classification. It is concluded that, for the anions, no simple model of enhanced mobility in a dislocation network (or grain boundaries) is in complete accord with all the known facts. It is suggested that the only reported autoradiographic experiment gives some evidence for a rather long-range effect of large-angle boundaries. By analogy, the external surface may cause abnormal distribution of vacancies throughout the region in which diffusion is usually studied.

We analyze the relation between maternal gradients and segmentation in Drosophila, by quantifying spatial precision in protein patterns. Segmentation is first seen in the striped expression patterns of the pair-rule genes, such as even-skipped (eve). We compare positional precision between Eve and the maternal gradients of Bicoid (Bcd) and Caudal (Cad) proteins, showing that Eve position could be initially specified by the maternal protein concentrations but that these do not have the precision to specify the mature striped pattern of Eve. By using spatial trends, we avoid possible complications in measuring single boundary precision (e.g., gap gene patterns) and can follow how precision changes in time. During nuclear cleavage cycles 13 and 14, we find that Eve becomes increasingly correlated with egg length, whereas Bcd does not. This finding suggests that the change in precision is part of a separation of segmentation from an absolute spatial measure, established by the maternal gradients, to one precise in relative (percent egg length) units.

Polyphysa peniculus was grown in artificial seawater in the presence of arsenate, arsenite, monomethylarsonate and dimethylarsinic acid. The separation and identification of some of the arsenic species produced in the cells as well as in the growth medium were achieved by using hydride generation-gas chromatography-atomic absorption spectrometry methodology. Arsenite and dimethylarsinate were detected following incubation with arsenate. When the alga was treated with arsenite, dimethylarsinate was the major metabolite in the cells and in the growth medium; trace amounts of monomethylarsonate were also detected in the cells. With monomethylarsonate as a substrate, the metabolite is dimethylarsinate. Polyphysa peniculus did not metabolize dimethylarsinic acid when it was used as a substrate. Significant amounts of more complex arsenic species, such as arsenosugars, were not observed in the cells or medium on the evidence of flow injection-microwave digestion-hydride generation-atomic absorption spectrometry methodology. Transfer of the exposed cells to fresh medium caused release of most cell-associated arsenicals to the surrounding environment.

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