DICT.TW Dictionary Taiwan

 polar body 名詞
 極體

 Po·lar a.
 1. Of or pertaining to one of the poles of the earth, or of a sphere; situated near, or proceeding from, one of the poles; as, polar regions; polar seas; polar winds.
 2. Of or pertaining to the magnetic pole, or to the point to which the magnetic needle is directed.
 3. Geom. Pertaining to, reckoned from, or having a common radiating point; as, polar coordinates.
 Polar axis, that axis of an astronomical instrument, as an equatorial, which is parallel to the earths axis.
 Polar bear Zool., a large bear (Ursus maritimus syn. Thalarctos maritimus) inhabiting the arctic regions.  It sometimes measures nearly nine feet in length and weighs 1,600 pounds.  It is partially amphibious, very powerful, and the most carnivorous of all the bears.  The fur is white, tinged with yellow.  Called also White bear. See Bear.
 Polar body, Polar cell, or Polar globule Biol., a minute cell which separates by karyokinesis from the ovum during its maturation.  In the maturation of ordinary ova two polar bodies are formed, but in parthogenetic ova only one.  The first polar body formed is usually larger than the second one, and often divides into two after its separation from the ovum.  Each of the polar bodies removes maternal chromatin from the ovum to make room for the chromatin of the fertilizing spermatozoon; but their functions are not fully understood.
 Polar circles Astron. & Geog., two circles, each at a distance from a pole of the earth equal to the obliquity of the ecliptic, or about 23° 28´, the northern called the arctic circle, and the southern the antarctic circle.
 Polar clock, a tube, containing a polarizing apparatus, turning on an axis parallel to that of the earth, and indicating the hour of the day on an hour circle, by being turned toward the plane of maximum polarization of the light of the sky, which is always 90° from the sun.
 Polar coordinates. See under 3d Coordinate.
 Polar dial, a dial whose plane is parallel to a great circle passing through the poles of the earth. --Math. Dict.
 Polar distance, the angular distance of any point on a sphere from one of its poles, particularly of a heavenly body from the north pole of the heavens.
 Polar equation of a line or Polar equation of a surface, an equation which expresses the relation between the polar coordinates of every point of the line or surface.
 Polar forces Physics, forces that are developed and act in pairs, with opposite tendencies or properties in the two elements, as magnetism, electricity, etc.
 Polar hare Zool., a large hare of Arctic America (Lepus arcticus), which turns pure white in winter.  It is probably a variety of the common European hare (Lepus timidus).
 Polar lights, the aurora borealis or australis.
 Polar opposition, or Polaric opposition or Polar contrast or Polaric contrast Logic, an opposition or contrast made by the existence of two opposite conceptions which are the extremes in a species, as white and black in colors; hence, as great an opposition or contrast as possible.
 Polar projection. See under Projection.
 Polar spherical triangle Spherics, a spherical triangle whose three angular points are poles of the sides of a given triangle. See 4th Pole, 2.
 Polar whale Zool., the right whale, or bowhead. See Whale.

 Pro·nu·cle·us n.; pl. Pronuclei   Biol. One of the two bodies or nuclei (called male and female pronuclei) which unite to form the first segmentation nucleus of an impregnated ovum.
 Note: ☞ In the maturing of the ovum preparatory to impregnation, a part of the germinal vesicle (see Polar body, under Polar) becomes converted into a number of small vesicles, which aggregate themselves into a single clear nucleus. which travels towards the center of the egg and is called the female pronucleus. In impregnation, the spermatozoon which enters the egg soon loses its tail, while the head forms a nucleus, called the male pronucleus, which gradually travels towards the female pronucleus and eventually fuses with it, forming the first segmentation nucleus.
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 polar body
      n : a small cell containing little cytoplasm that is produced
          along with the oocyte and later discarded