Young chick embryos were incubated on media containing formamide at concentrations of 0·1,0·25, 0·31,0·37, 0·43 and 0·5 M. Tn the neuroepithelium of these embryos we found that (1) the 0·1 M concentration had no detectable effect, (2) the 0-25 M concentration only affected mitosis which was blocked in metaphase so that mitotic figures accumulated near the neuro-coele, (3) 0·31 M formamide totally inhibited interkinetic nuclear migration and affected only slightly the cell asymmetry, (4) the 0·37 M concentration considerably reduced the amount of cytoplasmic microtubules and that the cells became round, (5) at 0·43 M formamide, all microtubules had disappeared and all cells were spherical, (6) at 0·5 M formamide all cells were spherical, detached from one another and the epithelium had lost its usual characteristics. Our results on exposure of the cells to low temperature (2 °C) suggest that formamide directly affects microtubules. All the effects observed at concentrations up to 0·43 M formamide are reversible.

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