The effect of wintering sites on the survival and reproduction of Gyraulus acronicus (Gastropoda) in a partly frozen river

Summary In a boreal river about 95% of the individuals of Gyraulus acronicus overwinter in the littoral zone which freezes solid each year. These snails were compared with those overwintering in the unfrozen sublittoral area: The littoral snails had a higher survival rate, a higher tissue dry mass/CaCO₃ ratio, and they deposited a higher number of eggs. Littoral snails had a more pronounced tissue degrowth during winter. High winter survival in the frozen littoral zone, a refuge totaly free from predation, indicates that overwintering here is advantageous. However, during frozen periods of short duration (<1 month) the high initial mortality to which the snails were exposed when freezing into the ice was not compensated for by higher survival after the initial phase. Under such conditions when the frozen period is very short the snails would have higher survival in unfrozen parts and are thus expected to avoid the ice.     

Overwintering of benthic macroinvertebrates in ice and frozen sediment in a North Swedish river

In winter the water freezes into the substrate within considerable areas of unregulated northern rivers due to low temperature combined with a lowering of the water level. Living individuals of Nematoda, Gastropoda, Sphaeriidae, Oligochaeta, Hirudinea, Isopoda, Trichoptera and Chironomidae were found in samples of ice and frozen sediment from the bottom frozen hydrolittoral zone of the north Swedish river Vindelälven. All abundant species in the frozen substratum, except Asellus aquaticus , seemed to be well adapted to withstand overwintering in this special habitat free from predation. Generally, between 80 and l00% of enclosed animals survived thawing. Cysts or other kinds of resting stage constructions, similar to those found during drought, were common in several enclosed species. Specimens of Gyraulus acronicus, Pisidium ssp., Molanna albicans and Chironomidae survived exposure to −4°C for five month in a freezing experiment. Extracellular freezing of the invertebrates overwintering in the ice is probable, as the ambient temperature was below the true freezing point of most animals. The composition of the substratum may effect the survival of animals enclosed in ice.     

Habitat, diet and food assimilation of Arctic charr under the winter ice in two subarctic lakes

The Arctic charr Salvelinus alpinus populations of the subarctic lakes Takvatn and Fjellfrøsvatn, north Norway, concentrated in the littoral zones (0–15 m) of the lakes during the entire winter (December to May) despite very low temperatures (0·2 and 0·7° C). High prey availability, low predation and competition and comparatively better light under snow and ice in shallow compared with deep water are probable reasons. At ice break in June, all Arctic charr moved to the profundal zone for a brief period, probably in response to the sudden light increase and a profundal resource peak of chironomid pupae. In the summer, the Arctic charr are found in the pelagic, profundal and littoral zones of the lakes. These populations therefore perform regular habitat shifts between the littoral zone in the winter, the profundal zone at ice break and the whole lake in the summer and autumn. The fish fed continuously during winter despite the cold water and the poor light. Amphipods and chironomid larvae dominated the diet. Catch per unit effort, numbers of stomachs with food and food intake rates varied with the subarctic light cycle but were lowest after the winter solstice. The winter assimilation of energy was about equal to the standard metabolism in Takvatn but was higher in Fjellfrøsvatn. The assimilation increased in both lakes under the spring ice in May. The habitat choice, diet and energy assimilation indicate that the Arctic charr is well adapted to the extreme winter conditions of subarctic lakes.     

Seasonal variation in the lateral distribution of mayfly nymphs in a boreal river

Seasonal variations in the lateral distribution of mayfly nymphs were investigated in the North Swedish river Vindelälven; a river with large seasonal differences in water level, current velocity and ice conditions, A total of 22 mayfly species about equally divided between summer and winter species were found along a single transect. At low water level in late summer and autumn most species were distributed over the whole transect. In winter the populations occupying the littoral moved into deeper water, so avoiding the ice. Only very small mortalities were detected in the ice. Prior to the spring flood most characteristically still water species moved to the shallow uppermost littoral while lotic species and the burrowing Ephemera vulgata L. did not show the same aggregation near the shore. It is suggested that these lateral movements allow the nymphs to avoid adverse environmental conditions and to exploit food resources over the whole transect.     

Evidence for Ideal and Non-Ideal Equilibrium Freezing of Leaf Water in Frosthardy Ivy (Hedera helix) and Winter Barley (Hordeum vulgare)

Temperature dependences of leaf water potentials (ψ_leaf) of frozen leaves of frosthardy ivy and winter barley were determined psychrometrically and found to coincide with the respective water potentials of ice which were obtained using the same technique. The water potentials of ice showed good agreement with theoretically established data. Analysis of the components of ψ of frozen leaves of Hedera helix revealed ideal equilibrium freezing, i.e. the governing of the relative content of liquid (or frozen) water solely by the osmotic potential. In winter barley, by contrast, a negative pressure potential was demonstrated to contribute to ψ_leaf. even under conditions of moderate frost. This reduced the degree of protoplast dehydration and the extent to which the concentrations of the cellular solutes rose. Such a freezing behavior is termed non-ideal equilibrium freezing. Depending on the original content of leaf water, the volume increments of liquid water due to the negative pressure potential amounted up to 10% at ?6 °C and even more at a lower temperature. In addition to the experimental data, a theoretical treatment of psychrometry at subzero temperatures is presented.     

Soil frost enhances stream dissolved organic carbon concentrations during episodic spring snow melt from boreal mires

Changes in winter time conditions at high‐latitude ecosystems could severely affect the carbon exchange processes. Using a 15 year stream record combined with winter field measurements and laboratory experiment, we studied the regulation of dissolved organic carbon (DOC) concentration in stream water draining boreal mire during snow melt. The most unanticipated finding was that cold soils with deep soil frost resulted in increased snow melt DOC concentrations in the stream runoff. Wintertime field measurements of DOC concentrations below the mire soil frost showed that this phenomenon could be explained by freeze‐out of DOC giving higher levels of DOC in the soil water below the ice as the soil frost developed downwards in the mire. Experimental freezing of water with a certain DOC concentration in the laboratory further corroborated the freeze‐out of DOC. In this experiment, as much as 96% of the DOC was excluded from the ice, whereas the freeze‐out in the mire was less effective (60%). The difference between the proportion of DOC retained in pure water relative to the proportion retained in peat water during freezing is probably due to trapped DOC in the solid peat soil matrix. A simple mass‐balance model showed that to explain the higher stream DOC concentrations during a winter with deep soil frost, approximately 0.5% of the mire area needed to be hydrologically connected to the stream discharge. In the stream records, we also found that the DOC concentrations during snow melt episodic runoff were negatively related to increasing intensity of the snow melt episodes (dilution by low DOC snow melt water) and higher previous export of DOC.     

Seasonal dynamics of akinetes of Anabaena flos-aquae in the bottom sediments and water column of a small Siberian reservoir

Seasonal dynamics of akinetes of Anabaena flos-aquae (Lyngb.) Breb. was studied in the sediments and the water column in profundal and littoral zones of a small Siberian reservoir. It was hypothesized that even in a shallow mixed reservoir cyanobacterial bloom initiates in the littoral zone and only then distributes throughout the water body. Two types of akinetes have been recognized: one type ensures vegetative reproduction, and the other, survival during adverse growth conditions. Seasonal dynamics of abundance of akinetes and vegetative cells was quite the same in water column in the littoral and profundal zones. However, there were differences in the seasonal dynamics of abundance of akinetes in the bottom sediments of these two areas: in the central zone the abundance increased throughout the vegetation period, whereas in the littoral zone the abundance decreased after settling of akinetes. During winter, the abundance of akinetes decreased in the sediments in both profundal and littoral zones. The explanation to this fact has not been found yet.     

Methane (CH4) release from littoral wetlands of Boreal lakes during an extended flooding period

Lake littoral zones have a transitional nature and dynamic conditions, which are reflected in their CH₄ emissions. Thus, detailed studies are needed to assess the littoral CH₄ emissions in a regional scale. In this study, CH₄ fluxes were followed during the ice‐free seasons in 1998 and 1999 by using the static chamber method in the littoral zone of two lakes in Finland. An exceptionally high water level in 1998 caused an unusually long inundation in otherwise ephemerally flooded zone. The flooding was normal in year 1999. The factors controlling CH₄ emissions were examined and statistical response functions were constructed. Further, the effect of extended flooding on the littoral CH₄ budged was estimated. The methane flux was primarily regulated by the water level in grass and sedge dominated eulittoral zone, but not in infralittoral reed and water lily stands. Methane emissions in the sedge dominated zone decreased significantly, when the flood was high enough to submerge the venting structures of the plants. Besides water level, sediment temperature determined CH₄ emission. The cumulative CH₄ emissions from the whole littoral wetlands in wet year were 1.1 times (L. Kevätön), or 0.61 and 0.79 times (L. Mekrijärvi) those in dry year. The crucial factor was the discrepancy between the exceptional and the average water level. The extension of inundated area does not necessarily increase CH₄ emissions if the flood reaches infrequently inundated areas, which apparently have low CH₄ production potential. This is the case especially, if the emissions in lower zones simultaneously decrease due to high water level. Our study analyses these complex responses between CH₄ emissions and water level.     

Ecosystem response to earlier ice break‐up date: Climate‐driven changes to water temperature,lake‐habitat‐specific production,and trout habitat and resource use

Climate warming has yielded earlier ice break‐up dates in recent decades for lakes leading to water temperature increases, altered habitat, and both increases and decreases to ecosystem productivity. Within lakes, the effect of climate warming on secondary production in littoral and pelagic habitats remains unclear. The intersection of changing habitat productivity and warming water temperatures on salmonids is important for understanding how climate warming will impact mountain ecosystems. We develop and test a conceptual model that expresses how earlier ice break‐up dates influence within lake habitat production, water temperatures and the habitat utilized by, resources obtained and behavior of salmonids in a mountain lake. We measured zoobenthic and zooplankton production from the littoral and pelagic habitats, thermal conditions, and the habitat use, resource use, and fitness of Brook Trout (Salvelinus fontinalis). We show that earlier ice break‐up conditions created a "resource‐rich" littoral–benthic habitat with increases in zoobenthic production compared to the pelagic habitat which decreased in zooplankton production. Despite the increases in littoral–benthic food resources, trout did not utilize littoral habitat or zoobenthic resources due to longer durations of warm water temperatures in the littoral zone. In addition, 87% of their resources were supported by the pelagic habitat during periods with earlier ice break‐up when pelagic resources were least abundant. The decreased reliance on littoral–benthic resources during earlier ice break‐up caused reduced fitness (mean reduction of 12 g) to trout. Our data show that changes to ice break‐up drive multi‐directional results for resource production within lake habitats and increase the duration of warmer water temperatures in food‐rich littoral habitats. The increased duration of warmer littoral water temperatures reduces the use of energetically efficient habitats culminating in decreased trout fitness.     

Physical and chemical features of an aestival pond in western Canada

Aestival ponds are shallow, semi-permanent water bodies that retain some water throughout the year, but freeze to the bottom during the winter. They are characteristically found at high latitudes or altitudes and present special conditions as environments for aquatic life. A study of such a pond in western Canada revealed two distinct seasons: an open-water season extending from March to November, during which water volume decreased considerably through evaporation, and a winter season in which all water remaining in the basin was frozen. Transitional periods between these two seasons were short. As water volume decreased during the summer, increases were noted in the specific conductance and hardness of the remaining water, whereas bicarbonate alkalinity was found to be directly correlated with rainfall alone. Values of other chemical constituents examined exhibited no distinct patterns. The formation of ice in early winter resulted in increases in all ion concentrations in the underlying water, just as occurs during the formation of sea ice. Furthermore, as the ice cover increased in thickness, the water beneath became supersaturated with dissolved oxygen. The consequences of this cryogenic “salting out”, are discussed.     

Sex differences in habitat distribution of a planktonic copepod, Eudiaptomus gracilis

The spatial distribution of adult males and females in a population of the copepod Eudiaptomus grachs ; was studied throughout one year The sexes were markedly segregated in spring and under ice in late winter This skewed distribution was mainly due to a strong concentration of females in deep water in the lake centre Males were more evenly distributed but dominated in relative terms strongly over females near the surface and the littoral zone Among females, those carrying eggs were most concentrated in deep regions Also during summer, females in the lake centre showed a marked abundance peak in deep water but the distribution of egg-carrying females did not differ from other females During autumn and early winter, differences in the distribution of the sexes were minor The distribution of the sexes is discussed in relation to predators, temperature, food, and mates Female copepods are susceptible to fish predation when carrying eggs Fish may contribute to the over-representation of females in deep regions by eating or scaring away this category of vulnerable prey from littoral areas and from the surface waters The fish predation hypothesis does not accurately predict the seasonal occurrence of the skewed distribution, however Feeding rate of fish is most likely low under ice Reproductive advantages in the warmer deep water may have contributed to female choice of habitat in late winter In spring and late winter proportionally fewer females near the surface and shore carried eggs, but they earned more spermatophores This indicates that these females were more receptive to matings Phytoplankton biomass was higher near the surface Thus, males may have gamed mating advantages and more food by avoiding deep water in the centre of the lake     

High-resolution satellite remote sensing of littoral vegetation of Lake Sevan (Armenia) as a basis for monitoring and assessment

Physics-based remote sensing in littoral environments for ecological monitoring and assessment is a challenging task that depends on adequate atmospheric conditions during data acquisition, sensor capabilities and correction of signal disturbances associated with water surface and water column. Airborne hyper-spectral scanners offer higher potential than satellite sensors for wetland monitoring and assessment. However, application in remote areas is often limited by national restrictions, time and high costs compared to satellite data. In this study, we tested the potential of the commercial, high-resolution multi-spectral satellite QuickBird for monitoring littoral zones of Lake Sevan (Armenia). We present a classification procedure that uses a physics-based image processing system (MIP) and GIS tools for calculating spatial metrics. We focused on classification of littoral sediment coverage over three consecutive years (2006–2008) to document changes in vegetation structure associated with a rise in water levels. We describe a spectral unmixing algorithm for basic classification and a supervised algorithm for mapping vegetation types. Atmospheric aerosol retrieval, lake-specific parameterisation and validation of classifications were supported by underwater spectral measurements in the respective seasons. Results revealed accurate classification of submersed aquatic vegetation and sediment structures in the littoral zone, documenting spatial vegetation dynamics induced by water level fluctuations and inter-annual variations in phytoplankton blooms. The data prove the cost-effective applicability of satellite remote-sensing approaches for high-resolution mapping in space and time of lake littoral zones playing a major role in lake ecosystem functioning. Such approaches could be used for monitoring wetlands anywhere in the world.     

Study on the Decomposition of Pistia stratiotes L. (Araceae) in Cisne Reservoir,Uruguay

A study on the decomposition process of Pistia stratiotes L. was carried out in Cisne Reservoir, Uruguay. For this purpose, leaves and roots were considered separately, and the process was studied in the littoral and the pelagic zone. The litter bag technique was used to estimate dry weight losses at different times. Leaves decomposed faster than roots in both zones. Pistia decomposed faster in the pelagic zone due to better oxygen conditions and a more intensive water movement that removes material from within the bags. Phosphorus, potassium, sodium and magnesium were leached rapidly during the first week. Nitrogen content in the leaves initially increased, reaching its maximum in the littoral zone that coincided with that of bacterial numbers colonizing the litter. The number of macroinvertebrates, dominated by Chironomidae, was low. A rough estimation of the phosphorus impact on the littoral zone associated to the decomposition process shows that after 24 h between 13.9 and 16.8 μg-at.1⁻¹ have been released to the water.     

Overwintering stress of Vaccinium vitis-idaea in the absence of snow cover

A snow manipulation experiment aimed to assess risks of direct freezing injury, freeze-induced dehydration and winter desiccation in the absence of snow cover on lingonberry (Vaccinium vitis-idaea). Frames with sheet-plastic sides and removable lids were used in this experiment for two purposes: to prevent accumulation of snow in mid-winter and to provide extra heat during early spring. Leaves were analyzed for frost hardiness, tissue water content and osmotic concentrations, and photoinhibition (Fv/Fm) during the period from the 10th of February to the 7th of April. The natural snow accumulation was low indicated by a minor difference in minimum temperatures between the frame treatment and naturally snow-covered plots. The heating effect of the frames started gradually at the end of February along with increasing solar elevation angles, and was highest at the beginning of April. Frost hardiness peaked in March as a consequence of cold periods, but it was practically lost by the beginning of April. Tissue water content decreased gradually at first, becoming greatly decreased later due to the extra heat. In accordance, the tissue osmotic concentrations increased first gradually, followed by a dramatic increase. Photoinhibition increased uniformly with increasing solar radiation, but at the end showed a sharp increment within a few days, obviously also indicating the effect of heating. It was concluded that neither lethal freezing stress nor significant freeze-induced dehydration occurred during the experiment. However, plants that overwintered without snow suffered from severe winter desiccation injuries due to the combination of solar heat and frozen soil. Although the desiccation stress was possibly a lethal factor, it was preceded by long-term and continued photoinhibition. It was concluded that during overwintering, chamaephyte species may suffer from both freezing and winter desiccation in the absence of protecting snow cover. However, during mild winters provided by climatic change scenarios, the risk of winter desiccation will be more probable. In relation to the future climate, it was concluded that winter desiccation and photoinhibition may develop gradually during a snowless winter and would, even if they did not reach a lethal level by themselves, possibly reduce frost hardiness.     

Development of phytoplankton in Lake Pääjärvi (Finland) during under-ice convective mixing period

The development of winter phytoplankton communities was studied in both shallow and deep areas of Lake Pääjärvi, southern Finland, during the final 2 weeks of winter ice cover. Phytoplankton was mainly composed of diatoms, cryptophytes and chrysophytes. The diatoms Aulacoseira and Rhizosolenia were always uniformly distributed with depth, initially probably due to mixing induced by heat flux from the sediment and later due to thermal convection. Motile Rhodomonas cryptophytes and Chrysococcus chrysophytes were most abundant near the ice showing that, despite their small size, they were partly able to resist mixing by convection. Their ability to stay in more illuminated water layers was reflected in net rates of increase about an order of magnitude higher than those of diatoms in the middle of the lake. Given the low temperatures and convection, the observed net rates of increase of motile taxa were very high compared to growth rates reported in the literature. The gradual increase in light availability following melting of ice led to a consistent increase in the abundances of major phytoplankton taxa irrespective of deep convective circulation. It is suggested that those algae most abundant at the time of ice break have a competitive advantage in the following open water conditions when nutrients are abundant but deep water circulation limits light availability. The results emphasize that in lakes which cool below the maximum density of water before freezing, apparently small differences in temperature and light conditions can cause important changes in the circulation patterns that impact on phytoplankton development.     

Methane in maritime Antarctic freshwater lakes

Summary Methane was found to occur in all freshwater lakes, irrespective of trophic status, sampled during this preliminary investigation at Signy Island, South Orkney Islands, Antarctica. Methane accumulated in the water column of these lakes during the winter period when ice cover prevented wind-induced mixing. Maritime Antarctic lakes are usually subject to wind-induced complete mixing during the summer open-water period but two major exceptions to the rule were found during this study. Methanogenesis occurred in both littoral and profundal regions of oligotrophic Sombre Lake. The presence of a substantial algal mat stabilized the Eh status of underlying sediments at the littoral site. Methane production was confined to the sediments in both littoral and profundal sediments during the study period (December–March) but in winter probably migrated to the sediment surface at the profundal site. All Signy Island lakes sampled were sulphate-poor and addition of sulphate markedly inhibited methanogenesis. Radio-isotope studies indicated that the H₂/CO₂ pathway was probably the predominant route for methanogenesis in these sediments through the acetate pathway appeared equally important at the sediment surface. In the absence of sulphate, sulphate reducers probably acted as net hydrogen donors to the methanogens. The process rate was permanently limited by the consistent low temperature (annual range 1–3°C). Rates increased with increasing temperature over the range 4–32°C, but no evidence was found to suggest cold sensitivity or psychrophily. The optimum temperature for methanogenesis was in excess of 30°C, temperatures never experienced at Signy Island. Rates of methanogenesis during the study period (Dec–Mar) ranged from 0.29 to 0.45 mg of carbon m^-2 and on an annual basis methanogenesis was calculated equivalent to 13% of the organic carbon deposition rate.     

Seasonal variation in vertical and horizontal movement of the freshwater bivalve Elliptio complanata (Mollusca: Unionidae)

1. Vertical and horizontal movement were studied in the freshwater bivalve Elliptio complanata at a sandy site in an oligotrophic lake over 3 years. Mussel movement did not vary significantly between day and night. On average, between 2 and 8% of 527 mussels moved each month during the ice-free season and the distance travelled by moving mussels averaged 0.6 cm day^–1.
2. Mussels were endobenthic during the winter, emerged from the sandy substrate in mid-May, peaked in sediment surface abundance in July, and descended into the sediments for the winter in September–October. Vertical displacement of mussels was closely correlated with water temperature although daylength may play a role. Mussels apparently move very little beneath the sediment during the winter.
3. The number of mussels moving horizontally at any given time was linearly correlated with daylength, but the distance travelled during a sampling period was related to daylength in a non-linear fashion. Greatest horizontal displacement of epibenthic mussels was found during spring and early summer, coincident with spawning in E. complanata     

Seasonal variation in vertical and horizontal movement of the freshwater bivalve Elliptio complanata (Mollusca: Unionidae)

1. Vertical and horizontal movement were studied in the freshwater bivalve Elliptio complanata at a sandy site in an oligotrophic lake over 3 years. Mussel movement did not vary significantly between day and night. On average, between 2 and 8% of 527 mussels moved each month during the ice-free season and the distance travelled by moving mussels averaged 0.6 cm day^–1.
2. Mussels were endobenthic during the winter, emerged from the sandy substrate in mid-May, peaked in sediment surface abundance in July, and descended into the sediments for the winter in September–October. Vertical displacement of mussels was closely correlated with water temperature although daylength may play a role. Mussels apparently move very little beneath the sediment during the winter.
3. The number of mussels moving horizontally at any given time was linearly correlated with daylength, but the distance travelled during a sampling period was related to daylength in a non-linear fashion. Greatest horizontal displacement of epibenthic mussels was found during spring and early summer, coincident with spawning in E. complanata     

Fish abundance and species richness across an estuarine–freshwater ecosystem in the Neotropics

We investigated the distributional patterns of shallow-water fish and their environmental correlates along a broad spatial scale encompassing estuarine and freshwater ecosystems. Marine-vagrant species were restricted to the sites under the influence of salinity intrusion, whereas estuarine-related and freshwater guilds were found along the entire estuarine–freshwater gradient. Primary- and secondary-freshwater fish guilds had the most widespread spatial distribution and comprised a major fraction of the total abundance and species richness. Abiotic factors correlated with fish abundance and composition along two main environmental axes, one related with variation in salinity, water transparency, and sediment granulometry and the other with the slope gradient. Species richness was significantly higher at the natural channel linking the estuarine- and freshwater-ecosystem, which probably was due to: (a) a steeper slope that favored the confluence of fish from the littoral (<2 m) and limnetic (>2 m) zones and (b) the sporadic inflow of saltwater that carried into this region several marine-related species. Although estuarine–freshwater ecotones are known to support few species, mainly salinity tolerant, our results suggest that habitat features and seasonal fish movement associated with salinity intrusion could lead to more diverse fish assemblages in this transitional zone.     

The management of extracellular ice by petioles of frost-resistant herbaceous plants

BACKGROUND AND AIMS: Some frost-tolerant herbaceous plants droop and wilt during frost events and recover turgor and posture on thawing. It has long been known that when plant tissues freeze, extracellular ice forms. Distributions of ice and water in frost-frozen and recovered petioles of Trifolium repens and Escholschzia californica were visualized. METHODS: Petioles of intact plants were cryo-fixed, planed to smooth transverse faces, and examined in a cryo-SEM. KEY RESULTS: With frost-freezing, parenchyma tissues shrank to approx. one-third of their natural volume with marked cytorrhysis of the cells, and massive blocks of extracellular icicles grew under the epidermis (poppy) or epidermis and subepidermis (clover), leaving these layers intact but widely separated from the parenchyma except at specially structured anchorages overlying vascular bundles. On thawing, the extracellular ice was reabsorbed by the expanding parenchyma, and surface tissues again contacted the internal tissues at weak junctions (termed faults). These movements of water into and from the fault zones occurred repeatedly at each frost/thaw event, and are interpreted to explain the turgor changes that led to wilting and recovery. Ice accumulations at tri-cellular junctions with intercellular spaces distended these spaces into large cylinders, especially large in clover. Xylem vessels of frozen petioles were nearly all free of gas; in thawed petioles up to 20 % of vessels were gas-filled. CONCLUSIONS: The occurrence of faults and anchorages may be expected to be widespread in frost-tolerant herbaceous plants, as a strategy accommodating extracellular ice deposits which prevent intracellular freezing and consequent membrane disruption, as well as preventing gross structural damage to the organs. The developmental processes that lead to this differentiation of separation of sheets of cells firmly cemented at determined regions at their edges, and their physiological consequences, will repay detailed investigation.