582 Chapter 18 • The Genetic Basis of Development any cell division, making the early embryo a synci-tium—a multinucleate cell. At nuclear division 9, a few nuclei migrate to the posterior pole where the polar granules are tethered. The plasma membrane of the syncitial egg then moves inward at the posterior pole to surround each nucleus, incorporating some of the polar granules into the cytoplasm that surrounds the nucleus. This action creates the pole cells, the ﬁrst mononucleate cells formed in the embryo. The pole cells will uniquely form the ﬂy’s germ line (Figure 18-7). How do the polar granules get tethered to the pos-terior pole of the oocyte? Again, one of the cytoskeletal networks carries the granules to the posterior pole and provides the rods to which the granules attach. In con-trast with C. elegans, in which the P granules attach to actin-based microﬁlaments, here the tubulin-based microtubules provide the essential trafﬁcking system. 18.3 Forming complex pattern:
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