In most species the cell cycle is arrested in the unfertilized egg. After fertilization the cell cycle is reestablished and a rapid series of cleavages ensues. Preceding the first cleavage in Xenopus the egg undergoes a contraction of its cortex, called the "surface contraction wave," which can be visualized by time-lapse cinematography. This wave of contraction is propagated in a circular manner from the animal pole to the equator. We have found that eggs prevented from cleaving by treatment with antimitotic drugs undergo a sequence of periodic surface contraction waves timed with the cleavage cycle in untreated eggs. In addition, artificially activated eggs, which fail to cleave presumably for lack of a functioning centriole, undergo the same periodic contractions. No nuclear material is required for the periodic waves because a separated egg fragment, produced by constricting a fertilized egg, still undergoes contraction waves with the same period as the cleaving nucleated fragment. These results demonstrate that some expression of the cell cycle persists in the absence of any nuclear material or centrioles, suggesting to us that a biological clock exists in the cytoplasm or cortex of vertebrate eggs, which may be involved in timing the cell cycle.In most species, the cell cycle is arrested in the unfertilized egg and is then reinstated by fertilization. In amphibians, after fertilization there is a somewhat long period leading up to the first cleavage and then cells in the animal, equatorial, and vegetal regions in sequence proceed through a rapid series of 10 or 11 metachronous cleavages until the midblastula stage is reached. During the metachronous cleavage period, the cells within each region divide synchronously. Thereafter, the cell cycle variably lengthens and cleavage becomes totally asynchronous (1-3). Thus, in amphibian development, the cell cycle is acquired in steps after fertilization, beginning with a short rudimentary cycle containing only M and S phases and characterized by little if any transcriptional activity and followed by a longer cell cycle having G1 and G2 phases and active RNA synthesis (4-6). Because the high degree of synchrony at the early stages is preserved when the individual blastomeres are dissociated and separated (ref. 2 and unpublished data), the machinery for timing accurately the alternate waves of DNA synthesis and mitosis must be partitioned to each of the daughter cells. An understanding of the mechanism of timing of the simple cell cycle in the early cleavage period may be useful for understanding the overall control of the cell cycle in somatic cells.Although many cell types have been used for studies of the cell cycle, the amphibian egg offers some special advantages. Its large size permits enucleation and nuclear transplantation, allowing a clear-cut distinction between the contributions of the nucleus and the cytoplasm. In general, the lack of activity of the nucleus in the early cleavage period makes the egg a good system for studying the autonomy of the...

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The distribution of 0‐group flatfish was investigated in 1992 in the Dollard (Ems–Dollard estuary, Wadden Sea). 0‐Group plaice, flounder and sole were not evenly distributed over the sampled locations. The spatial distribution pattern of 0‐group flatfish in the Dollard changed during the investigation period. In the first week of sole presence, when the mean length of sole was 24–30 mm, salinity correlated significantly with sole density. The distribution of juvenile sole larger than 40 mm total length was affected by the elevation of the location: 0‐group sole was restricted to the sampled site with the lowest elevation. The distribution of 0‐group plaice was related to sediment: no juvenile plaice were caught at locations with more than 10% mud fraction in the sediment. The distribution of 0‐group flounder was also correlated with sediment. Later in the year, salinity correlated negatively with the distribution of 0‐group flounder. The influence of sediment composition is probably indirect and linked to the abundance of preferred food items, such as Corophium volutator. Abiotic conditions were suitable to 0‐group plaice, flounder and sole.

Shortly after local artificial insemination, but well before egg rotation,Xenopus eggs show a wave-like propagation of dark-light-dark zones from the site of sperm entrance. This presumably reflects the movement of the front of cortical granule breakdown.

ProblemDunes, beaches and surf zones have always protected the land against flooding by the sea. However, a sandy coast is more than just a pile of sand providing a natural defence against the sea. The protection of the Dutch coastal environment is laid down in (inter)national legislation and regulations, such as the EU Water Framework Directive (WFD), the EU Bird and Habitat Directive, and in international treaties and recommendations. Moreover, in 2006, the European Commission adopted the Marine Strategy Directive to protect the marine environment; it states that management of coasts should be based on an ecosystem approach (http://ec.europa.eu). To protect the coastal environment, one has to know what to protect and (i) what are its characteristics (ii) which organisms inhabit it and (iii) which functions does it fulfil? The surf zone of beaches is, however, rarely accessible for faunal sampling because of harsh and dynamic wave climate.On the Dutch coast, we assume that the main shortterm threats are sand nourishment to counter erosion, the mechanical cleaning of beaches, and disturbance by vehicles and tourists -all consequences of increasing recreational pressures, pollution and fisheries. Important Abstract Surf zones are highly dynamic, physically stressful parts of sandy beach ecosystems. The high wave energy of surf zones has in the past severely hampered ecological surveys of these systems. Here we used a novel technique to collect fauna from this environment along the Dutch coast. A large vehicle in the form a tripod that drives along the sandy seafloor and supports a sampling platform 11 m above the water line can collect both infaunal (grabs) samples and pull beam trawls for epibenthos. The distribution and diversity of macrofauna were studied at different depths in the surf zone along the Dutch coast. Species diversity and abundance increased with increasing depth of the water column. This increase was especially noticeable on the seaward side of the outer breaker bar. Within the surf zone, in the trough between the two breaker bars, there were spots of high diversity and abundance of macrobenthic infauna. Moreover, the area is also important for epibenthic and fish species, like the commercially important flatfish sole. Spatial patterns of species richness and abundance across an onshore-offshore gradient from the beach to seawards of the breakers suggest the presence of faunal zonation in this environment. The high abundance recorded in troughs was primarily caused by patches of juvenile Sand mason Lanice conchilega. The management implications of these results are that we suggest to protect the surf zone, including the trough between the two breaker bars, as a potential area of high diversity and abundance and to reconsider the objectives of the EU-Habitat Directive and the Water Framework Directive for the coastal area.

In fertilizedXenopus eggs, shortly after egg rotation but well before the occurrence of the cleavage-associated "surface contraction waves", two circular dark zones originate consecutively from the pigment spot marking the site of sperm entrance. They expand and travel centrifugally over the egg surface.

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