egg cleavage

Segmentation \Seg`men*ta"tion\, n.
   The act or process of dividing into segments; specifically
   (Biol.), a self-division into segments as a result of growth;
   cell cleavage; cell multiplication; endogenous cell
   formation.
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   {Segmentation cavity} (Biol.), the cavity formed by the
      arrangement of the cells in segmentation or cleavage of
      the ovum; the cavity of the blastosphere. In the gastrula
      stage, the segmentation cavity in which the mesoblast is
      formed lies between the entoblast and ectoblast. See
      Illust. of {Invagination}.

   {Segmentation nucleus} (Biol.), the body formed by fusion of
      the male and female pronucleus in an impregnated ovum. See
      the Note under {Pronucleus}.

   {Segmentation of the ovum}, or {Egg cleavage} (Biol.), the
      process by which the embryos of all the higher plants and
      animals are derived from the germ cell. In the simplest
      case, that of small ova destitute of food yolk, the ovum
      or egg divides into two similar halves or segments
      (blastomeres), each of these again divides into two, and
      so on, thus giving rise to a mass of cells (mulberry mass,
      or morula), all equal and similar, from the growth and
      development of which the future animal is to be formed.
      This constitutes {regular segmentation}. Quite frequently,
      however, the equality and regularity of cleavage is
      interfered with by the presence of food yolk, from which
      results unequal segmentation. See {Holoblastic},
      {Meroblastic}, {Alecithal}, {Centrolecithal},
      {Ectolecithal}, and {Ovum}.

   {Segmentation sphere} (Biol.), the blastosphere, or morula.
      See {Morula}.
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Egg \Egg\ ([e^]g), n. [OE., fr. Icel. egg; akin to AS. [ae]g
   (whence OE. ey), Sw. [aum]gg, Dan. [ae]g, G. & D. ei, and
   prob. to OSlav. aje, jaje, L. ovum, Gr. 'w,o`n, Ir. ugh,
   Gael. ubh, and perh. to L. avis bird. Cf. {Oval}.]
   1. (Popularly) The oval or roundish body laid by domestic
      poultry and other birds, tortoises, etc. It consists of a
      yolk, usually surrounded by the "white" or albumen, and
      inclosed in a shell or strong membrane.
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   2. (Biol.) A simple cell, from the development of which the
      young of animals are formed; ovum; germ cell.
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   3. Anything resembling an egg in form.
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   Note: Egg is used adjectively, or as the first part of
         self-explaining compounds; as, egg beater or
         egg-beater, egg case, egg ladle, egg-shaped, etc.
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   {Egg and anchor} (Arch.), see {egg-and-dart} in the
      vocabulary, below; -- called also {egg and dart}, and {egg
      and tongue}. See {Anchor}, n., 5. --Ogilvie.

   {Egg cleavage} (Biol.), a process of cleavage or
      segmentation, by which the egg undergoes endogenous
      division with formation of a mass of nearly similar cells,
      from the growth and differentiation of which the new
      organism is ultimately formed. See {Segmentation of the
      ovum}, under {Segmentation}.

   {Egg development} (Biol.), the process of the development of
      an egg, by which the embryo is formed.

   {Egg mite} (Zo["o]l.), any mite which devours the eggs of
      insects, as {Nothrus ovivorus}, which destroys those of
      the canker worm.

   {Egg parasite} (Zo["o]l.), any small hymenopterous insect,
      which, in the larval stage, lives within the eggs of other
      insects. Many genera and species are known.
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