(Seg"ment) n. [L. segmentum, fr. secare to cut, cut off: cf. F. segment. See Saw a cutting
1. One of the parts into which any body naturally separates or is divided; a part divided or cut off; a section; a
portion; as, a segment of an orange; a segment of a compound or divided leaf.
2. (Geom.) A part cut off from a figure by a line or plane; especially, that part of a circle contained between
a chord and an arc of that circle, or so much of the circle as is cut off by the chord; as, the segment acb
in the Illustration.
3. (Mach.) (a) A piece in the form of the sector of a circle, or part of a ring; as, the segment of a
sectional fly wheel or flywheel rim. (b) A segment gear.
4. (Biol.) (a) One of the cells or division formed by segmentation, as in egg cleavage or in fissiparous
cell formation. (b) One of the divisions, rings, or joints into which many animal bodies are divided; a
somite; a metamere; a somatome.
Segment gear, a piece for receiving or communicating reciprocating motion from or to a cogwheel,
consisting of a sector of a circular gear, or ring, having cogs on the periphery, or face. Segment of
a line, the part of a line contained between two points on it. Segment of a sphere, the part of a
sphere cut off by a plane, or included between two parallel planes. Ventral segment. (Acoustics)
See Loor, n., 5.
(Seg"ment) v. i. (Biol.) To divide or separate into parts in growth; to undergo segmentation,
or cleavage, as in the segmentation of the ovum.
1. Relating to, or being, a segment.
2. (Anat. & Zoöl.) (a) Of or pertaining to the segments of animals; as, a segmental duct; segmental
papillæ. (b) Of or pertaining to the segmental organs.
Segmental duct (Anat.), the primitive duct of the embryonic excretory organs which gives rise to the
Wolffian duct and ureter; the pronephric duct. Segmental organs. (a) (Anat.) The embryonic excretory
organs of vertebrates, consisting primarily of the segmental tubes and segmental ducts. (b) (Zoöl.)
The tubular excretory organs, a pair of which often occur in each of several segments in annelids. They
serve as renal organs, and often, also, as oviducts and sperm ducts. See Illust. under Sipunculacea.
Segmental tubes (Anat.), the tubes which primarily open into the segmental duct, some of which
become the urinary tubules of the adult.
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 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
(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.