On the possibility of sperm aster involvement in dorso-ventral polarization and pronuclear migration in the amphibian egg

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Abstract
Sperm aster involvement in the determination of the plane of bilateral symmetry of the future embryo as well as pronuclear movement and orientation has been investigated in the amphibian egg. The procedure involved injecting very small amounts of disrupted sperm cells into the subcortical region of the egg of the toad Bufo arenarum. Using this procedure we have previously shown that structural integrity of the sperm cell is not required for the determination of the symmetry plane. The present report deals with the cytological effects induced by the injection of sperm homogenate into the egg. As expected, an aster was always present in these eggs. This aster closely resembled that produced in the fertilized egg as the sperm nucleus migrates through the egg mass. Sections of injected eggs have also shown the presence of two pigment trails similar in appearance to those produced by pronuclear movement after normal fertilization. The streak analogous to the sperm entrance track emerged from the point where the pipette was introduced. None of these effects were observed in Ringer’s injected eggs. At the time of the rotation of symmetrization, the female pronucleus was seen within a clear area of cytoplasm located at a short distance below the egg cortex. An aster field organized around this clear area was probably formed as a result of the egg centriole activation. In no case was the egg nucleus seen to migrate towards the copulation site as was seen to occur in sperm homogenate-treated eggs. The active agent was found to be a particulate component which sediments with the head but not the tail fraction of Bufo arenarum sperm. Injection of blood cells caused a similar response to that elicited by the sperm homogenate.