The possible mechanism of cell positioning in mouse morulae: an ultrastructural study.

Ultrastructure of the mouse morula at the time of formation of the outer cell layer is described. Analysis of relative positions of sister blastomeres, still connected with cytoplasmic bridge, suggests that cell division may occur both tangentially and perpendicularly in relation to the embryo surface. However, there are observations suggesting that active cell movement may be involved in cell positioning already after mitosis. These observations include: (1) the flexure of cytoplasmic bridge, (2) the presence of bundles of filaments in external blastomeres at the late cytokinesis and (3) a striking difference in loose versus tight packaging of blastomeres within the embryo.