Tuesday, June 30, 2009
In animal development, organogenesis (organo-genesis, compound of the Greek words όργανον "that with which one works", and γένεσις "origin,creation,generation") is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. Internal organs initiate development in humans within the 3rd to 8th weeks in utero.The germ layers in organogenesis differ by three processes: folds, splits, and condensation. Developing early during this stage in chordate animals are the neural tube and notochord. Vertebrate animals all differentiate from the gastrula the same way. Vertebrates develop a neural crest that differentiates into many structures, including some bones, muscles, and components of the peripheral nervous system. The coelom of the body forms from a split of the mesoderm along the somite axis.
Monday, June 29, 2009
Wednesday, June 24, 2009
Tuesday, June 23, 2009
Monday, June 22, 2009
Arcology, from the words "architecture" and "ecology," is a set of architectural design principles aimed toward the design of enormous habitats (hyperstructures) of extremely high human population density. These largely hypothetical structures, called "arcologies," would contain a variety of residential and commercial facilities and minimize individual human environmental impact. They are often portrayed as self-contained or economically self-sufficient.
Sunday, June 21, 2009
A system with hysteresis can be summarized as a system that may be in any number of states, independent of the inputs to the system. To be exact, a system with hysteresis exhibits path-dependence, or "rate-independent memory". By contrast, consider a deterministic system with classical dynamics but no hysteresis. In that case, one can predict the output of the system at some instant in time, given only the input to the system at that instant. If the system has hysteresis, then this is not the case; one cannot predict the output without looking at the history of the input, i.e., the state of the system for a given input. In order to predict the output, one must look at the path that the output followed before it reached its current value.
Many physical systems naturally exhibit hysteresis. A piece of iron that is brought into a magnetic field retains some magnetization, even after the external magnetic field is removed. Once magnetized, the iron will stay magnetized indefinitely. To demagnetize the iron, it would be necessary to apply a magnetic field in the opposite direction. This effect is exploited commercially; for example, it provides the element of memory in a hard disk drive.
Saturday, June 20, 2009
Antinatalism is the philosophical position that asserts a negative value judgment towards birth. It has been advanced by figures such as Arthur Schopenhauer, Brother Theodore and David Benatar. Schopenhauer, in his essay On the Suffering of the World articulates the position as follows:
Similarly, Benatar argues from the hedonistic premise that the infliction of harm is generally morally wrong and therefore to be avoided, and the intuition that the birth of a new person always entails nontrivial harm to that person, that there exists a moral imperative not to procreate.
If the act of procreation were neither the outcome of a desire nor accompanied by feelings of pleasure, but a matter to be decided on the basis of purely rational considerations, is it likely the human race would still exist? Would each of us not rather have felt so much pity for the coming generation as to prefer to spare it the burden of existence, or at least not wish to take it upon himself to impose that burden upon it in cold blood?
Friday, June 19, 2009
Thursday, June 18, 2009
Wednesday, June 17, 2009
Detritivores, also known as detritus feeders or saprophages, are heterotrophs that obtain nutrients by consuming detritus (decomposing organic matter). By doing so, they contribute to decomposition and the nutrient cycles.
Detritivores are an important aspect of many ecosystems. They can live on any soil with an organic component, and even live in marine ecosystems where they are termed interchangeably with bottom feeders.
Tuesday, June 16, 2009
Monday, June 15, 2009
Sunday, June 14, 2009
Saturday, June 13, 2009
Thursday, June 11, 2009
Wednesday, June 10, 2009
Monday, June 8, 2009
Sunday, June 7, 2009
Saturday, June 6, 2009
Friday, June 5, 2009
A community Eruv (Hebrew: ערוב mixture, also transliterated as Eiruv or Erub, plural: Eruvin) refers to the legal aggregation or "mixture" under Jewish religious property law of separate parcels of property meeting certain requirements into a single parcel held in common by all the holders of the original parcels, which enables Jews who observe the traditional rules concerning Shabbat to carry children and belongings anywhere within the jointly held property without transgressing the prohibition against carrying a burden across a property line on the Jewish sabbath. The legal aggregation is set up to have effect on the Sabbath day only; on other days of the week, including Yom Tov (with the exception of Yom Kippur), ordinary property ownership applies. A valid aggregation has a number of requirements including an agreement among the property-holders and an aggregation ritual.
One of the requirements of a valid aggregation is that all the parcels must lie within a chatzer, or walled courtyard. For this reason, this type of aggregation is more properly known as an eruv chatzerot (Hebrew: ערוב חצרות), an "aggregation of courtyards," to distinguish it from other types of rabbinically-ordained mixture procedures which also have the name eruv.