The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

blastocoel \blastocoel\, Blastocoele \Blas"to*c[oe]le\
blastocoele \blastocoele\, n. [Gr. blasto`s sprout + koi^los
   hollow.] (Biol.)
   The cavity inside a blastula; -- also called segmentation
   cavity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

segmentation cavity
    n 1: the fluid-filled cavity inside a blastula [syn:
         blastocoel, blastocoele, blastocele, segmentation
         cavity, cleavage cavity]