The embryonic development of the polyclad flatworm Imogine mcgrathi

In this paper we describe the embryonic development of the polyclad flatwor m Imogine megrathi. Imogine is an indirect developer that hatches as a plan ctonic Goette's larva after an embryonic period of approximately 7 days. Li ght and electron microscopic analyses of sections of staged embryos were co mbined with antibody stainings of wholemounted embryos to reconstruct the o rigin and movement of the primordia of the various organ systems, with part icular emphasis on the nervous system. We introduce a system of morphologic ally defined stages aimed at facilitating future studies and cross-species comparisons among flatworm embryos. Imogine embryos undergo typical spiral cleavage. Micromere quartets 1-3 form an irregular double layer of mesenchy mal cells that during gastrulation expands over micromere quartet 4. Microm ere 4d divides into several large mesendodermal precursors whose position d efines the ventral pole of the embryo. These cells, along with the animal m icromeres that obtained a sub-surface position during cleavage, form a deep layer of cells that gives rise to all internal structures, including the n ervous system, musculature, nephridia, and gut. Micromeres 4a-c are large y olky cells that are incorporated into the lumen of the gut, but do not them selves contribute to the gut epithelium. Shortly after gastrulation, cell d ifferentiation sets in. Cells located at the surface adopt epithelial chara cteristics and form cilia that result in continuous movement of the post-ga strula stage embryo. Deep cells at the lateral margins of the embryo become organized into a protonephridial tube. A cluster of approximately 50 deep cells at the anterior pole forms the brain, in which we have identified set s of founder neurons of the brain commissure and the dorsal and ventral con nectives. The early differentiating neurons, along with other cells forming stabilized microtubules (ciliated cells of the epidermis, gut and protonep hridia; apical gland cells) could be analyzed in detail because of their la beling with an antibody against acetylated alpha-tubulin. Our findings indi cate that, despite significant differences in the cleavage pattern and arra ngement of blastomeres in the early embryo, morphogenesis and organ formati on of a polyclad embryo follows a pattern that is very similar to the patte rn observed by us and others in phylogenetically more evolved rhabdocoel fl atworms.