This paper discusses the results of microscopic examination on the histological process of fertilization by the interspecific cross pollination in the combinations of Pinus thunbergii Parl. with P. densiflora Sieb. et Zucc. and several pine species. The work of pollination was carried out in 1968 at Kamigamo Experimental Forest Station of Kyoto University. In order to make clear the process in the development of ovule, the strobili were collected at definite intervals, then they were fixed by bouin's fluid. Ovules were cut to serial sections by rotary microtome and they were fixed as parmanent preparation by usual method. As the results of anatomical examination on the development of ovule, female and male gametophytes were normal to develop in the intraspecific cross pollination of P. thunbergii ×P. thunbergii as the control cross for this examination and in three interspecific pollinations of P. thunbergii×P. densiflora, ×P. massoniana Lamb., ×P. luchuensis Mayr.. Full seeds were obtained by these intra- and interspecific pollinations, when the seeds matured. It, therefore, means that both developments of male and female gametophytes have been normal. By these pollinations in which male and female gametophytes developed nomally, it was possible to grasp principal tendency toward the stage in the development of ovule, but with a little early or late among samples. It was, namely, observed that the endosperm was formed within late May to early June at about 400 days after pollination, then the archegonia was formed. And the egg cell was examined in late June at about 410 days after pollination. On the other hand, the path of the pollen tube through the nucellus was observed in late June at about 415 days after pollination and the pollen tube invated into the archegonia in late June, so that the fertilization would be seen soon. Contrary to above-mentioned, other four interspecific pollinations in the combinations of P. thunbergii×P. banksiana Lamb., ×P. pinaster Ait., ×P. rigida Mill. and ×P. taeda Linn. did not produce any full seed. The failure in such combinations signifies that a development of male gametophyte within the term from pollinaton to fertilization was abnormal, despite a development of female gametophyte was normal. A sterile phenomenon in these pollinations, neither a pollen could germinate nor a pollen tube could elongate, was observed at very early time at one month after pollination. Although there will be various inferences in a reason of causing this sterile phenomenon, it seems that some genetic factors of specific characters or chemical difference between ovules of these species may be related to a control reaction for the germination of pollen or the elongation of pollen tube.