イエコウモリ

Histological studies on the matu ration and fertilization of mammalian ovum have dealt chiefly with rodents but few in Chiroptera; that is, on Vesperugo noctula by Van der Stricht ('09), on Myotis lucifugus lucifugus by Wimsatt ('44) and on Corynorhinus rafinesquei intermedius by Pearson, Koford and Pearson ('52). Investigation of the Japanese house bat, Pipistrellus tralatitius abramus (Temminck), was made after the previous report (Uchida '50). 1). The ovarian egg containing the resting nucleus before and in the period of hibernation, enters the diffuse stage after awakening from hibernation. The process of the first and second maturation division and that of the formation and conjugation of both pronuclei in this bat are similar to that in other mammals. 2). As compared with ot her mammals, Graafian follicle shows special conditions, and the ovum is surrounded by a large mass of cumulus cells which consist of more than ten layers. The ovum is thrown out in the periovarian space, surrounded by the cumulus cells, and the fertilization is assumed to take place here. 3). The mating takes place chiefly on and after the last decade of October and the numerous sperms remain in the uterus until the ovulation occurs in April. However, very few sperms arrive at the tube or periovarian space at the ovulation time so that the existence of sperms can be scarcely noticed by the histological examination. 4). It seems that the denudation of the ovum by the sperms before fertilization is not a prerequisite of fertilization in vivo, at least in this animal, because the cumulus cells scarcely isperse after fertilization in the greater part of cases examined. It appears that the delay of the dispersion of follicle cells has something to do with the fact that the layer of cumulus cells is very thick and that very few sperms approach the ovum at the ovulation time. 5). The facts that the ovulation occurs on the last decade of April, and that the number of ovulated ova, as well as the number of corpora lutea, is always less than three, give the histological evidences for the conclusion of the previous report. Moreover, taking into consideration that the size of the litter ranges from one to three, we may assume a high fertilization rate. 6). Judging from the facts that the ovum is protected by the thick layer of cumulus cells, that the sperms exist in the uterus for a long period, and that the fertilization takes place in the periovarian space which is situated nearer to ovary than ampulla, we assume that the fertilization is more sure to take place, and the fertilization rate is higher than in other cases. Together with the ecological peculiarity, series of these particular reproductive phenomena may be considered as an adaptation for the racial preservation of this species in which the fertility is rather lower.