Fig. 6

Embryos stained with DAPI to show the distribution of nuclei at four successive stages of development in C. fornicata. The formation of the expansive provisional epithelium, comprised of a small number of cells (nuclei) can be seen in (d, f, and h). a-b Corresponding fluorescence micrographs showing the vegetal (a) and animal (b) sides, respectively in a round stage embryo undergoing early stages of epiboly. The large opening of the blastopore is seen in (a), which is sparsely populated with cells, as the progeny of the animal micromeres have not yet made it to the vegetal side. Notice the fairly symmetrical pattern of the cells (nuclei) in the animal hemisphere in (b). c-d Corresponding fluorescence micrographs showing the vegetal (d) and animal (d) sides, respectively, of a flattened embryo undergoing later stages of epiboly. c The blastopore has closed considerably. d Notice that cells are somewhat more dispersed on the future dorsal side of the embryo as the provisional epithelium is beginning to form. e-f Corresponding fluorescence micrographs showing the ventral and dorsal sides, respectively, of an embryo undergoing elongation. The provisional epithelium is forming on the dorsal surface. g-h Corresponding fluorescence micrographs showing the ventral and dorsal sides, respectively, of an older embryo that has started organogenesis. h The provisional epithelium is present on the dorsal surface of the head, and the shell gland has begun to form in the post-trochal region. Additional nuclei appear to be present in the region of the provisional epithelium, as the deeper endodermal macromeres have undergone some divisions by this stage of development. a, anterior; bp, blastopore; d, dorsal; P, posterior; pe, provisional epithelium; sg, shell gland; st, stomodeum; V, ventral. Asterisk in b, d, e-h marks the location of the animal pole. Scale bar in h equals 50 μm