The results of the scanning electron microscopy are summarised in the table. Ectoderm ceils from the hydroxyureatreated embryos were different from ectoderm cells from normal embryos, since after 3 rain in culture they showed no filopodia (figure 3). After 0.5 h in culture the cells remained featureless and had not flattened onto the glass substrate, and after 3 h the cells had not formed pseudopodia. Endoderm cells from the hydroxyurea-treated embryos were also different from endoderm cells from normal embryos, since they were featureless at all times of culture and did not show pseudopodia at 3 h. Since it was possible that cells were featureless because they were moribund, cell viability was determined by trypan blue exclusion. Cells were prepared as for electron microscopy, and after 3 rain and 3 h of culture they were exposed to 1% trypan blue in dissociation medium for 10 rain. The results confirmed that at least 87%, and usually 95%, of the cells from both control embryos and hydroxyurea-treated embryos remained viable. Thus cells from embryos which were prevented by hydroxyurea treatment from undergoing normal morphogenesis did not show the features and changes in vitro shown by cells from normal embryos. 3. Cells cultured in hydroxyurea: An additional experiment was performed to determine whether hydroxyurea had any direct effect on the appearance of Xenopus embryonic cells.Ectoderm cells were dissected from normal embryos, and dissociated and cultured in the presence of 10 -2 M hydroxyurea. Cells after 3 rain in culture with hydroxyurea (that had been exposed to hydroxyurea during dissociation) were seen to be different from cells from both normal and hydroxyurea-treated embryos -many ectoderm cells showed smooth 'blebs' or 'bulges' (figure 4). After 0.5 h in culture in the presence of hydroxyurea many of the cells were featureless, and a few still showed 'blebs'; after 3 h more of the cells were featureless (table). This finding might suggest that hydroxyurea has 2 effects on cells from Xenopus early embryos; a more immediate effect which produces 'blebs', followed by a longer-term effect which prevents the formation of surface features such as filopodia and pseudopodia. These results suggest that cells from embryos which are prevented from undergoing normal morphogenesis at gastrulation show a deficiency in their properties in vitro, and are compatible with the idea that the study of cells isolated from early embryos and cultured in vitro may increase our understanding of the mechanisms and controls of morphogenesis.1 Acknowledgments. We wish to thank Mr C. Veltkamp for his help with the electron microscopy, and Mrs J. Clumpus for technical assistance.Summary. Polytene chromosomes in salivary gland nuclei from chironomid larvae of defined stages are connected along their entire length, by some 100 chromatin fibers, to the inner nuclear membrane.Chromatin forms structural attachments on the inside of the nuclear envelope, which may serve to hold the interphase chromosomes in fixed positions relative to...
