Our knowledge of the early development of Man has made marked progress during recent years, numerous fairly well-preserved embryos having been described in greater or less detail. There are still, however, numerous gaps in our knowledge, pertaining not merely to points of detail but to fundamental questions, so that any early embryo reasonably well preserved and reasonably well sectioned is deserving of careful study. Our specimen belongs to the comparatively late presomite stage represented in the literature by such embryos as Kl13 (Grosser, 1913), Wa17 (Grosser, 1931), Peh.
1
-Hochstetter (Rossenbeck, 1923), and the embryo of Ingalls (1918), and characterised by the presence of an elongated primitive streak, a luminated chorda-process and a prochordal plate. We offer no apology, however, for presenting a fairly detailed description of yet another example of this stage, since our embryo amongst other things provides a more complete picture of the cranial region of the head-process than any specimen hitherto described.
Early efforts at modeling wetland ecosystems were aimed primarily at reflecting biomass or nutrient dynamics. A number of models have been developed for different wetland types, including coastal salt marshes, mangrove wetlands, freshwater marshes, swamps, and riparian wetlands. The early ecosystem models were mostly simple compartment models with linear, constant-coefficient differential equations used to simulate biomass or nutrient dynamics. Practically no contaminant flux was incorporated into these models. With few exceptions, the ecosystems were considered spatially homogeneous. At the same time that the ecosystem models were being developed, considerable effort was given to modeling various wetland processes, such as circulation and sediment transport. Other process-level modeling included plant and animal uptake and elimination of both organic chemicals and heavy metals. The level of detail in these process models, however, has not been applied to most ecosystem models. There has been a recent trend, however, to increase the complexity of ecosystem-level models and to incorporate spatial dynamics. These developments should greatly enhance the ability to simulate contaminant transport and effects in wetlands.
-Early efforts at modeling wetland ecosystems were aimed primarily at reflecting biomass or nutrient dynamics. A number of models have been developed for different wetland types, including coastal salt marshes, mangrove wetlands, freshwater marshes, swamps, and riparian wetlands. The early ecosystem models were mostly simple compartment models with linear, constant-coefficient differential equations used to simulate biomass or nutrient dynamics. Practically no contaminant flux was incorporated into these models. With few exceptions, the ecosystems were considered spatially homogeneous. At the same time that the ecosystem models were being developed, considerable effort was given to modeling various wetland processes, such as circulation and sediment transport. Other process-level modeling included plant and animal uptake and elimination of both organic chemicals and heavy metals. The level of detail in these process models, however, has not been applied to most ecosystem models. There has been a recent trend, however, to increase the complexity of ecosystem-level models and to incorporate spatial dynamics. These developments should greatly enhance the ability to simulate contaminant transport and effects in wetlands.
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