The embryonic development of the temnocephalid flatworms Craspedella pedumand Diceratocephala boschmai

We have analyzed the embryonic development of the temnocephalid flatworms C raspedella pedum and Diceratocephala boschmai, using a combination of fuchs in-labeled whole-mount preparation, histology, and transmission electron mi croscopy. Following the staging system recently introduced for another flat worm species (Mesostoma lingual, we can distinguish eight morphologically d efined stages. Temnocephalids produce eggs of the neoophoran type in which a small oocyte is surrounded by a layer of yolk cells. Cleavage takes place in the center of the yolk mass (stages 1-2) and results in an irregular, m ultilayered disc of mesenchymal cells that moves to the future ventral egg pole (stage 3). Organ primordia, including those of the brain, pharynx, mal e genital apparatus, sucker, and epidermis "crystallize" within this disc w ithout undergoing gastrulation movements (stage 4). An invagination of the epidermal primordium pushes the embryo back into the center of the yolk ("e mbryonic invagination"). As a result, organogenesis begins while the embryo is invaginated (stage 5). The brain differentiates into an outer cortex of cell bodies that surround a central neuropile. Precursor cells of the epid ermis, pharynx, and protonephridia become organized into epithelia. During stage 6, the embryonic primordium everts back to the surface, where organog enesis and cell differentiation continues. Epidermal cells fuse into a sync ytium that expands around the yolk. Myoblasts initially do not spread out i n the way epidermal cells do; they remain concentrated in two narrow, longi tudinal bands that extend along the sides of the embryo. Three pairs of axo n tracts extending posteriorly from the brain follow the bands of myoblasts . Stages 7 and 8 are characterized by the appearance of eye pigmentation, b rain condensation, and the formation of tentacles and a sucker that bud out from the epidermis of the anterior and posterior end, respectively. Compar ison of morphogenesis in temnocephalids with observations in other flatworm taxa suggests a phylotypic stage for this phylum of invertebrates.