[Orientation of the cleavage spindles in pulmonate mollusks. II. The role of the architecture of the intercellular contacts in III and IV cleavage spindle orientation].

In the normal development of pulmonate molluscs, the variety of orientations of the III and IV cleavage spindles markedly reduces in the process of transition from pro- to meta- and anaphase. Even prior to the completion of spindle growth in these cleavage divisions the external faces of blastomeres become asymmetrical due to intercellular interactions but the whole system of external intercellular contacts (edges) is characterized by a certain symmetry. This symmetry coincides partially or fully with that of the system of definitive spindles. In the artificially obtained four-celled common pond snail embryos with the chain-like position of blastomeres, the III cleavage spindles were oriented at right angles to the polar axis, rather than in parallel with it (as in the normal development). The eight-celled embryos with symmetrical external faces of macromeres were also obtained. The variety of orientation of the IV cleavage definitive spindles in such embryos was markedly widened and in the macromeres with inverse asymmetry the inversion of the sign of declination of the spindles was observed. The spindle orientation depends, thus, on the form of adjacent region of the external face. This form as a whole is determined by the mutual position of blastomeres, curvature of surface and relative length of the face edges.