Genetic material and information passed on from individuals of one generation to the next. Genetic material and hereditary are not synonymous; for example, a fertilized egg contains much cytoplasm that is non-genetic. Also, material passed from the mother to the embryo via the placenta is in a sense hereditary, but again not genetic. See Behavior …
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Her Majesty’s Inspectorate (HMI)
Founded in the late 19th century to act as the ‘eyes and ears’ of the British government with regard to state-funded schooling. It was terminated and replaced by the Office for Standards in Education (Ofsted) at the end of the 20th century when the British government adopted an interventionist stance over schooling. See Education
Hemoglobin
A pigmented protein (or metelloprotein) that contains iron in the form of heme located in red blood cells of vertebrates and responsible for the delivery of oxygen (about 98% of the total) to tissues (heme being the deep red, non-protein, ferrous component of hemoglobin). The color of blood, which varies from bright red when oxygenated …
Hensen’s node
Named after Victor Hensen (1835-1924), and also called the primitive knot, it is the thickened area at the anterior end of the primitive streak or groove through which gastrulating cells invaginate to form the notochord. Found in birds, mammals and reptiles, it is homologous to the dorsal lip of the blastopore in amphibian embryos, and …
Hedonism
The doctrine, rooted in Greek philosophy, that the pursuit of pleasure is the chief good. Thus, a hedonist strives to maximize pleasure (minus any pain). See Pleasure-pain principle
Hebbian learning
A method of weight adaptation in connectionist models that is biologically plausible. Weights between two units get strengthened if both units are active at the same time (“what fires together wires together”). Several variations of Hebbian learning exist (e.g., involving weight decay and temporal aspects of unit activation). See Cell assemblies, Computational models, Connectionist models, …
Hebbian synapse
A basic mechanism for synaptic plasticity, posited to underlie learning and memory, through which an increase in synaptic efficacy arises from the persistent stimulation of the postsynaptic cell by the presynaptic cell. Thus, when an axon of cell A is close enough to excite cell B and is repeatedly involved in firing it, some growth …
Hawthorne effect
The effect of simply being studied, usually improving performance no matter what changes are made to the circumstances. The effect is often attributed to being the subject of focused observation. It is named after the Hawthorne works of the Western Electric Company in Chicago where seven studies were carried out on the effects of changing …
Hazard ratio
A unit-free ratio of the odds of onset of a given outcome in any given unit of time for individuals with a given risk, divided by the odds of onset over the same period for individuals without such a risk. Put another way, it is the ratio of the instantaneous probability of the outcome event …
Haws
Small, apple-like fruits of a large group of shrubs and small trees with thorny branches in the genus Crataegus, family Rosaceae. They are important for many nectar-feeding insects.