{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}.