ABSTRACT
In an attempt to explain the determination mechanism of embryonic polarity, the relation between the behaviour of ectoplasm and the turning of the P2-cell in embryo sof Parascaris equorum, Rhabditis ikedai and Rhabditis sp., has been studied by means of centrifugation.
During cleavage of uncentrifuged eggs, extension and contraction of the cell-surface occur. These are accompanied by streaming of the ectoplasm. In the early phase of the second cleavage embryos become T-shape. Along with streaming of ectoplasm at the animal side of S2-cell in the later phase, the surface of S2-cell extends on one side and contracts on the other. Successively, P2-cell turns from the extending side of S2-cell to the contracting one, that is, in the direction of the primary streaming of ectoplasm. Thus, the embryos become rhomboidal in shape and their axes are established. The extended side of S2-cell points roughly to the ventral side of the embryo, and the other to the dorsal.
In the centrifuged embryos, extension and contraction of cell-surfaces and turning of P2-cell take place also accompanied by streaming of the ectoplasm at the centrifugal side of S2-cell.
It is concluded from these facts that the determination of embryonic polarity depends on the turning of the P2-cell by the extension and the contraction of the surfaces of S2-cell and that the direction of this turning depends on that of the primary streaming of ectoplasm in S2-cell. It is assumed that the direction of the streaming is due to the migration of the nucleus, and that the extension and contraction of cell-surfaces is based on the behaviour of the E.R. and microtubules in the ectoplasm. The tetrahedral embryo is caused by a change in the streaming of ectoplasm. The formation of a rhomboidal embryo in Rhabditis without a preceding T-stage is discussed in connection with the behaviour of the ectoplasm.