Fig. 4. Relocalisation of the germ granule during segmentation in Sagitta inflata. The germ line blastomere and the germ granule are coloured in two shades of green. (A,B) The germ granule (arrowhead) is cortical and juxtaposed to the cleavage plane during stages 2 (A) and 4 (B). (C-E) The sequence C to D shows the migration of the germ granule (arrowhead) towards the nucleus (N) during stage 4-8. During telophase in E, the germ granule is seen stretching towards one of the spindle poles. It is inherited by one of the two blastomeres at stage 8. (F) Stage 8. As in E, the germ granule (arrowhead) is lying on the side of the spindle and is closer to one of the spindle poles (compare to E). (G,H) Stage 16. The germ granule (arrowhead) forms a cap around one spindle pole (G; live observation in DIC) (H; thick section tangential to vegetal pole). (I) Stage-31 cells. All blastomeres have divided except for the larger cell containing the germ granule (light green). (J) Stage-32 cells. Unequal cleavage segregates the germ granule (arrowhead) into the smallest blastomere constituting the founder PGC. (K) Stage-32 cells. The germ granule cannot be seen in vivo but the founder PGC is identifiable because it remains in interphase with a large visible nucleus. (L) Stage-63 cells. All blastomeres divide except for the founder PGC. (M) Stage 64 cells. The founder PCG has divided into two cells identifiable by the continuous presence of an interphase nucleus. (N) Blastula stage.
