In some strains of mice, eggs when X irradiated during the pronuclear stage, undergo a mitotic block in the G2 phase of the first cell cycle and cleave when the second division takes place in controls. The importance of this effect varies considerably with the strain and depends exclusively on the maternal genotype. In previous work, two-dimensional electrophoresis showed that eggs blocked at the one-cell stage after irradiation, undergo the same modifications in polypeptide synthesis as two-cell controls of the same age, except at the time of normal first mitosis, where three polypeptide sets of 30, 35 and 45 kDa appear only in cleaving controls. In the present study, we have found phosphorylations in dividing controls, on polypeptides of 30, 35 and 45 kDa. These phosphorylations are not seen in blocked irradiated eggs.

The effects of cycloheximide (CH) on grey crescent formation in artificially maturedAmbystoma mexicanum oocytes were determined. CH induced grey crescent formation after a few hours, especially after a 45° to 90° rotation from the vertical animal-vegetal axis. With low concentrations of CH (about 0.5 ng/oocyte), meiosis was still able to proceed normally to the stable second metaphase stage, but higher concentrations blocked it after 1st polar body extrusion and an interphasic nucleus appeared. Such effects were compared to those of inactone, an analogue of cycloheximide, which as a pure substance does not inhibit protein synthesis, but still contained a small amount of CH in the available samples. It is concluded that grey crescent formation can occur in non-activated oocytes. The effects of cycloheximide might be due to partial inhibition of protein synthesis and the presence of a proteinic inhibitor of the symmetry reaction in the normal oocyte is suggested.