[Orientation of the cleavage spindles in pulmonate mollusks. I. The role of the form of the blastomeres in 2d cleavage spindle orientation].

In the normal two-celled embryos of various pulmonate molluscs, the orientation of spindles characteristic of metaanaphase is being frequently established gradually, in the process of transition from pro- to metaphase accompained by the growth of spindle and asters. The typical growth of contact zone between the blastomeres of the common pond snail embryos was inhibited to a different extent under their cultivation after the 1 cleavage division in the calcium-free media or after trypsinization. At the same time the orientation of meta-anaphase spindles was markedly affected (as judged by an angle alpha between the spindle axis and the plane of contact zone in the equatorial projection). When analyzing the model distributions of the angles between the two spindle axes (in the same projection), it was shown that the empirical distributions of these angles corresponded to the principle of stochastic combination of two alpha. A conclusion is drawn that the orientation of one spindle does not depend on that of another but the position of each of them depends on the size of the contact zone and, hence, on the general form of the adjacent blastomere region. Some other processes determining the spindle orientation are discussed.