Cell lineage of the midline cells in the amphipod crustacean Orchestia cavimana (Crustacea, Malacostraca) during formation and separation of the germband

Cell lineages of identified midline cells were. traced in the amphipod Orch estia cavimana (Crustacea, Malacostraca) by in vivo labelling. Midline cell s are at common phenomenon in the germ band of crustaceans and insects. Stu dies in midline cells of Drosophila showed an origin from separate, paired anlagen and a differentiation into three types of cells. The in vivo labell ing of midline cells of Orchestia demonstrates that they originate from the same material as the neural and epidermal ectoderm, divide in a stereotype d cell division pattern and give rise to at least two different types of ce lls. During the following evolutionarily derived mode of germ band elongati on in Orchestia, a morphogenetic process is intercalated that separates ger m band halves. On the level of single cells, it can be shown that midline c ells are the only ectodermal cells that bridge the large distance between t he separated parts. The cells are stretched extensively but do not prolifer ate. Comparing the midline cells of Orchestia with non-malacostracan crusta ceans and insects, the results favour the hypothesis that midline cells are a distinct population of cells homologous in crustaceans and insects.