Fig. 2

Schematic diagram highlighting cells/clones derived from the first quartet micromeres 1a¹-1d¹ and 1a²-1d². These cells/clones are color coded as indicated in the key. a-c Animal pole views with the d quadrant located towards the bottom of the figure. The other micromere progeny are labeled in (a) for reference. The central small round polar bodies (gray color) are located at the animal pole. Note that the 1q² cells do not undergo subsequent divisions and initially present a very small exposed area on the surface of the embryos, but eventually they spread out to occupy a greater exposed surface area (also see Fig. 3). d-g Diagrams showing later stages and differential movement of the 1q² micromeres to form the provisional epithelium, with progeny of 1d¹, on the dorsal side of the head. Color has been removed from the 1q¹ clones for better clarity. These rearrangements help accommodate the relative displacement of the animal pole towards the vegetal/ventral pole during development. The posterior pole is located towards the bottom of the figure for embryos shown in (d-g). d Dorsal view during late cleavage in an embryo undergoing compaction. e Dorsal view of an embryo beginning elongation. f Dorsal view of an older embryo that is beginning organogenesis. The circular, condensed shell gland (sg) has begun to form in this embryo. g Right lateral view of an embryo that is somewhat older than that in (f). Note the animal pole ends up as the anterior pole of the embryo/larva while the vegetal pole (the site of gastrulation, blastopore, and mouth) ends up on the ventral surface. The originally straight animal-vegetal axis becomes bent, and the animal pole becomes located at 90 degrees relative to the vegetal/ventral pole. For each embryo, the animal/anterior pole (A) is indicated with a green-headed pin. The vegetal/ventral pole (V) is indicated with a red-headed pin in g. Embryos in (a-c) follow illustrations of Conklin [40]. Embryos in (d-g) are from confocal images where the cell outlines are visualized by expression of GFP-tagged UTPH (from [29])