The ultrastructure of the pinealocytes of noctule bats, mammals which live most of the time in darkness or very low light intensities, was examined and compared with the pinealocytes of other mammals. Two different populations of pinealocytes (I and II) were observed. They differ in general aspect, in location and especially in their content of cell organelles involved in synthetic processes. Mitochondria, ribosomes, lysosomes and lipid inclusions were present in the perikaryon of pinealocytes of both populations. In the pinealocytes of population I some granular vesicles, of presumed Golgi origin, and some other structures were observed. Pinealocytes of population II are characterized by many glycogen granules, more or less associated with a large vacuolar system. Moreover, some small vacuoles originating from cisterns of the granular endoplasmic reticulum and containing flocculent material of a moderate electron density are described. The possibility is discussed that these small vacuoles are involved in one of the secretory processes of the pineal gland while the granular vesicles of the pinealocyte of the population I are the products of another.

1. The activity cycle in a colony of about 150 pond bats is studied throughout the summer season in Berlikum (Netherlands). The natural diurnal roosts of the bats are sited between rafters of a church loft where a nocturnal darkness reigns day and night. The onset of activity generally takes place in two phases: I-descending into and waiting in a narrow exit chamber from where daylight can be seen, II-flying out to the feeding grounds. The investigation includes electronic recording of passages of bats, and of light intensity during morning and evening twilight. 2. Phase I is regulated by an endogenous circadian timing system which is synchronised to the seasonal variation in daylength. The main entraining agent is daylight which the fats can only have observed, either when waiting in the exit chamber at dusk on the preceding day, or when flying home at dawn. The large time lapse occurring between the moments of arrival of first and last descender in the exit chamber (2 hrs), and between the return of the first and last homeflyer (1 hr) on a given day, is ascribed to lack of precision in timing of the right moment. This lack is thought to originate from two facts. First, the almost dayly variation of cloudiness of the sky at the time of departure causes the rate of decrease of daylight to fluctuate irregularly. Second, at the times of departure and return most of the bats are confronted with very low light intensities only being poor entraining agents. In addition, there is some evidence that each individual has an oscillatory entrained activity cycle causing it to be among the first descenders (homeflyers, resp.) at one day, among the last descenders (homeflyers, resp.) at another day. Some relevant literature data on responsecurves concerning other nocturnal mammals are discussed. 3. The seasonal phase relationship between the daylight cycle and the actiiity rhythm observed in pond bats accords with the Aschoff-Wever model, but only as far as the light-dark ratio is concerned. Twilight duration does not seem to be an important factor in this respect. The value of light intensity at the moment of departure of the first outflyer decreases as the season proceeds. 4. Phase II (the flying out) is regulated grosso modo by the absolute value of light intensity at the moment of departure. It is made plausible that the releasing stimulus for departure of the first outflyer, on clear evenings, is the surpassing of a threshold value of the rate of relative decrease of light intensity. 5. Literature data on activity cycles of other bat species are discussed.

An electron microscopical investigation of the pineal gland of the adult noctule bat revealed the presence of some peculiar ciliary derivatives, similar to the club-shaped outer segment of rudimentary photoreceptor cells in the pineal organ of nonmammalian vertebrates. The pinealocytes of population I can be classified in several morphological types, one of them displaying morphological features resembling those of rudimentary photoreceptor cells. These results reconfirm the concept of the sensory cell line in the vertebrate pineal organ. The question whether the pinealocytes of population II belong to the same sensory cell line is discussed.

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