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.
 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 n.
 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.
 2. Biol. A simple cell, from the development of which the young of animals are formed; ovum; germ cell.
 3. Anything resembling an egg in form.
 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.
 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 Zoöl., any mite which devours the eggs of insects, as Nothrus ovivorus, which destroys those of the canker worm.
 Egg parasite Zoöl., any small hymenopterous insect, which, in the larval stage, lives within the eggs of other insects. Many genera and species are known.