Mechanism of cleavage of newt eggs.

Cytokinesis by contraction of the base of the cleavage furrow and the dependence of the position of the future furrow on the arrangement of cytoplasm under it have been demonstrated at relatively earlier stages of cleavage in amphibian eggs. The following experiments and observations on dividing eggs of the newt Cynops pyrrhogaster indicated the presence of the contracting force at the base of the furrow and revealed that the above cleavage mechanisms occurred throughout cleavage. (1) Deformation of the animal surface induced conversion of an early furrow to a ridge, the base of the furrow forming the top of the ridge. (2) A needle placed in the path of the furrow blocked its deepening, not only at the point of contact but also in neighbouring regions. Any part of the plane of cleavage could be blocked in this way. (3) A microfilamentous band was found under the plasma membrane at the base of the furrow at a late stage of cleavage. (4) Alteration in the position of the furrow base could be induced in the middle of cleavage by outflow of egg cytoplasm.