The temporary occurrence of the callose plate between the generative and vegetative cells in pollen grains was observed in 82 angiosperm species and 4 gymnosperm species. The period of isolation of the generative cell by callose, determined as the “callose stage”, includes the early interphase of the generative nucleus. The significance of the “callose stage” in the differentiation process of generative cells is discussed. The opinion is expressed that the temporary occurrence of a callose plate may explain why some authors only observe a double plasma membrane separating the generative and vegetative cells of the pollen grain, while others observe a wall-like structure between these membranes.