NEW OBSERVATIONS ON THE EMBRYOLOGY OF THE TARDIGRADA

A reinvestigation of tardigrade embryology is presented. The species s tudied were the eutardigrade Halobiotus crispae and the heterotardigra de Echiniscoides sigismundi. Nomarski and TEM techniques were used and the embryos were reconstructed three dimensionally whenever possible. Cleavage was found to be total and equal. The pattern of cleavage cou ld not be determined, since it proved impossible to determine the orie ntation of the embryos. Cleavages were asynchronous from the second cl eavage onwards and gastrulation, which was by delamination, took place at a very early stage in development. The mode of mesoderm formation could not be determined, but the presence of mesodermal cells before t he occurrence of coelomic pouches is rendered probable. The structure of yolk grains and egg chorion is described. The organogenesis of the digestive tract, the cuticular structures, and the nervous tissues is described. Primordia of all major organs were present from approximate ly day 7 after egg laying in H. crispae. Maturation continues after ha tching. It is concluded that the cleavage pattern could be derived fro m the spiral cleavage pattern. It is unlikely that the mesoderm is for med from the coelomic pouches, which are interpreted as the primordia of the limb mesoderm in this study. The findings presented are compare d to earlier papers on the subject.