Fig. 3

Formation of the four quartets in M. crozieri. a–g SEM pictures coloured according to micromere quartets. a First quartet (1Q and 1q indicated in blue). b–d Second quartet (2Q and 2q) indicated in green. e Third quartet (3Q and 3q) indicated in orange. f, g The large fourth micromere quartet (4q) are shown and indicated in yellow. The fourth quartet micromeres are shown in red. h Closeup of the fourth quartet micromeres (4A–4D) with Phalloidin staining (red) outlining their cell shape. Nuclear staining (blue) is DAPI. i–l Formation of the fourth quartet. i The 16-cell stage shows macromeres 3B-D and their nuclei at an animal position within the large blastomeres. j Same embryo as in G but at the 32-cell stage. Nuclei of 3B and 3D are now positioned at the vegetal pole of the macromeres. k 33-cell stage of a 3D reconstructed embryo (Their depth in the embryo is coded by colours as seen in top right part of the panel. Division of one of the four macromeres (3Q) into 4Q/4q has taken place. The white arrow indicates the newly formed small macromere of the fourth quartet (4Q) coloured purple indicating it is close to the vegetal pole. l 3D reconstructions showing that all four macromeres comprising the fourth quartet are now positioned at the most vegetal pole of the embryo (coloured purple and indicated by arrows). Scale bar sare 50 μm