Ultrastructure of how18 mutant CBs. (A–C) Ultra-thin sections of how18 mutant embryo at late stage 17. (B and C) High magnifications of A showing the J domains at both sides of the embryo (yellow arrows). Note that CBs are very round compared with the wild type (compare A with Fig. 3 A) but L domains are conserved. However, the lumen does not swell and shows numerous circumvolutions. Abbreviations and annotations are the same as in Fig. 3. Bars, 1 μm.