Fig. 2. Visualization of the CAB in posterior vegetal blastomeres of Phallusia mammillata embryos. (A-D) Live Phallusia embryos viewed by DIC optics (A-C) or by fluorescence confocal microscopy (D). (A) Images extracted from a time-lapse sequence (see Movie 1 in supplementary material). t=0: the interphase nucleus (n) appears pinched (arrow) as it migrates towards the CAB; t=5' (5 minutes): the CAB (arrowhead in all panels) appears as a smooth zone, which compacts and thickens during prophase; t=9' (9 minutes): the spindle positions asymmetrically, causing unequal cleavage into large (B6.4) and small (B6.3) cells. (B) The CAB is visible as a smooth cortical zone (arrowhead) late in the cell cycle. (C) A transient surface protrusion forms at the position of the CAB early in the cell cycle. (D) Embryo double labelled for ER (red) and mitochondria (green). The nucleus (n) and centrosome (c) are oriented toward the cER-rich CAB. (E) In situ hybridization showing localization of a postplasmic/PEM mRNA in the CAB (antisense probe for Phallusia Vasa). (F) Embryo fixed with formaldehyde viewed by DIC optics. (G) Scanning electron micrograph shows surface structures protruding at the position of the CAB. The names of the relevant blastomeres are indicated as drawn in Fig. 1. Bars, 10 µm.