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Fig. 9 Immunofluorescent staining of microtubules in untreated (panels A-D) and UVtreated embryos (panels E-F). (A) An untreated embryo, approximately 30 minutes after fertilization. When examined at high magnification (C), a parallel array of microtubules can be seen in the yolk cytoplasmic layer. (B) An untreated embryo at approximately 3.5 hours after fertilization, with a spreading yolk syncytial layer (ysl). Numerous microtubules radiate from the yolk syncytial layer (ysl) into the yolk cytoplasmic layer (ycl), as shown at high m a g n i fication in D. Microtubules are disrupted in UV-treated embryos (E,F). In embryos fix e d 10 minutes after irradiation, microtubules throughout the embryo appear abnormal. (E) One such embryo, where microtubules in the yolk cytoplasmic layer are either very sparse (arrow) or present in the aberrant form of ‘comet-tails’ (arrowhead). In embryos fixed 2 hours after irradiation, an example of which is shown in F, some microtubules appear normal, as demonstrated by the mitotic aster in the yolk syncytial layer (arrowhead). In the yolk cytoplasmic layer, however, microtubules are still disrupted: there are far fewer microtubules in the region bordering the yolk syncytial layer (compare with similar region in panel D). Bar, 250 7mu;m (for A,B); 50 μm (for C-F).