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Egg cleavage
(gcide) | 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 cleavage
(gcide) | 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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