Dictionary,Defintion,Meaning,What is Pacemaker, Palaeozoic, Pacinian Corpuscle, Palaeozoic, Palate, Palea, Palmate, Palmella, Palolo Worm

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Home >>Biology Dictionary >> Pacemaker - Palaeozoic

	P₁ (Parental generation). Consisting of the individuals from which a breeding experiments start, all the males being uniform as regards the allelomorphes under observation and all the females like-wise. Pacemaker.(1) (sinoatrial node) A small mass of specialized muscle cells in the mammalian heart, found in the wall of the right atrium near the opening for the venacava. The cells initiate and maintain the heart beat : by their rhythmic and spontaneous contractions; they stimulate contractions in the rest of the heart muscle. These cells themselves are controlled by the automatic nervous system which determines the rate. Similar pacemakers occurs in the hearts of other vertebrates. (2) An electronic or nuclear battery-charged device that can be implanted surgically into the chest to produce and maintain the heart beat. These devices are used when the heart’s own pacemaker is defective or diseased.
Pachytene. The period in the first prophase of meiosis when paired homologous chromosomes are fully contracted and twisted around each other. Pacinian corpuscle. A heavy pressure receptor is mammalian skini. Paedogenesis. Reproduction in larval or other pre-adult form. Pairing. (of chromosome (synapsis). Side by side association of homologous chromosome at meiosis. Palaeo. Denoting old or prechistoric.
Palaeobotany. Study of fossil plants. Palaeoecology. The study of the ecology of part environment through fossil organism. Palaeolithic. Phase of human history during which, through tools were manufactured, food was obtained solely by hunting fishing, or collecting, with no cultivation. Lasted from at least half a million years ago up to beginning of Neolithic stage about 10,000 years ago. Palaeontology. Study of fossils Palaeozoic. Geological era lasting approximately from 570 till 225 million years ago. (See Geological Periods and Epochs.) Palate. Roof of vertebrate mouth. In mammals and crocodiles roof of mouth is not homologous with that of other vertebrates; a new (false) palate has developed beneath original palate, by bony shelves projecting inwards from bones of upper jaw. Palea. A Glunee-like bract found on the axis of individual flowers in grasses.
Palmate. Having at least three leaflets joined to the Petiole at the same place. Palmella. Stage or growth from of certain algae, in which numerous cells are embedded with a gelatinous matrix. Palolo Worm. (1) In some burrowing polychaete worms modified sexual forms swarm to the ocean surface for communal liberation of eggs and sperm. Some such cases of ‘Palolo worms’ occur with marked periodicity in tune with lunar and diurnal cycles (e.g. in a West Indian species, at 3 = 4a.m during the first or last quarter of the lunar cycle in July) giving rise to a spectacular event exploited by human and other worm eaters. (2) Originally referred to a Samoan species.

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