In the early cleavage stages of animal embryos, blas-tomeres lack devices to connect to each other. It is well known that in sea urchin embryos the hyaline layer plays an important role in maintaining the position of individual blastomeres, and provides a scaffold for morphogenesis to the embryo. In starfish embryos, however, the presence of the hyaline layer is not certain in the early cleavage stages, although it has been observed at the gastrula stage (Dan-Sohkawa et al., 1986). In the present study we have investigated the devices corresponding to the hyaline layer of sea urchin embryos in the early cleavage stages, where blastomeres lack such devices for cell adhesion as desmosomes.

We examined how blastomeres keep their position in the early cleavage stages of starfish embryos using Asterina pectinifera and Astropecten scoparius. By neither electron microscopy nor immunofluorescent staining with an antibody against the hyaline layer of sea urchin embryos (Yazaki, 1968) could we detect the hyaline layer, at least up to 6 h after fertilisation.

When the fertilisation envelope (FE) was dilated larger by urea treatment, blastomeres increasingly came apart with the expansion of the FE, resulting in the formation of plural small blastulae due to the failure of the blastomeres to come together into a single mass. In urea-treated embryos, blastomeres were observed closely apposed to the inner surface of the FE. These observations suggest that blastomeres are fixed by some means to the FE. Differential interference-contrast microscopy revealed numerous projections between the cell surface of the blastomeres and the FE. Probably, the FE and the projections are involved in maintaining the three-dimensional position of each blastomere in an embryo.