A | B | C | E | G | H | I | L | M | N | O | P | S | T | V | Y | Z |


Abembryonic Pole: A pole that is away from the embryo.

Amniogenesis: The development of the amnion.

Amniotic Cavity: The fluid filled space within the cavity that is surrounded by amnion. A layer of extraembryonic ectoderm which encloses the embryo with a fluid filled cavity.

Ampulla: of the uterine tube: the thin-walled, almost muscle-free, midregion of the uterine tube; its mucosa is greatly plicated.

Anastomosis: A connection between two vessels.

Antitragus: A projection opposite the tragus, bounding the cavum conchae posteroinferiorly and continuous above the anthelix.

Auricular Hillocks: Embryonic tubercles adjoining the first branchial groove that gives rise to the auricle of the ear.


Bilaminar: Having or pertaining to two layers, as the basement membrane that comprises the basal lamina and the reticular lamina.

Blastoceole: This fluid filled space forms inside the embryo four days after fertilization.

Blastocyst: The mammalian conceptus in the post-morula stage. It has a fluid filled cavity known as blastoceole and is not limited to one germ layer.

Blastomere: One of the cells produced by cleavage of a fertilized ovum.

Branchial Arches: The arches extend ventrally and laterally around the pharynx in order to support the gills of lower vertebrates. The absence of gills in higher vertebrates causes these arches to be known as pharyngeal arches.

Branchial Grooves: The first bronchial groove is the only one that develops into an adult structure - external and auditory meatus remaining II-IV grooves eventually disappear and the ridge thus becomes smooth.


Chorion: The cellular, outermost extraembryonic membrane, composed of trophoblast lined with mesoderm. It develops villi about two weeks after fertilization. Villi are the threadlike projections growing in tufts on the external surface of the chorion. At three weeks, it is vascularized by allantoic vessels.

Coalescence: The fusion or blending of parts.

Concha: a shell; used in anatomical nomenclature to designate a structure or part that resembles a shell in shape.

Cytotrophoblast: The cellular (inner) layer of the trophoblast, surrounds the inner all mass.


Ectodermal: Pertaining to or derived from the ectoderm. The ectoderm is the outermost of the three primary germ layers of the embryo. From it are developed the epidermis and the epidermal tissues, such as the nails, hair, the nervous system, external sense organs such as the ear and eye.

Endolymphatic: Pertaining to the fluid contained in the membranous labyrinth of the ear.

Endodermal Cells: The innermost cells of the three germ layers in the embryo. From it are derived the epithelium of the pharynx, respiratory track (except the nose), the digestive tract, bladder, and urethra.

Endometrial: Pertaining to the endometrium. The endometrium is the inner mucous membrane, the thickness and structure changing with the phase of the menstrual cycle. It is functionally divided into three layers: the stratum basale, stratum spongiosum, and stratum compactum.

Exocoelomic Membrane: A delicate sac of mesoblastic tissue that develops as a lining of the blastocyst or chorionic cavity just after implantation, forms the exocoelomic cavity and quickly disappears.


Germ Disc: A flattish area in a cleaved ovum in which the first traces of the embryo are seen.

Glossopharyngeal: Pertaining to the tongue and pharynx.


Hensons Node: A primative knot; a mass of cells at the cranial end of the primative streak, related to the organization of the embryo. It is the functional equivalent of the dorsal lip of the amphibian blastopore.

Hyoid Bar: A pair of cartilaginous plates forming the second visceral arch, from which a part of the hyoid bone is developed.


Inner Cell Mass: Group of centrally located blastomeres, thin outer layers of cells


Lacuna: A small pit or hollow cavity.

Lumbar: Pertaining to the loins, the part of the back between the groin and the pelvis.

Lumbosacral: Pertaining to the loins and the sacrum.


Male: The nuclear material of the head of the spermatozoon, after it has penetrated the ovum and acquired a pronuclear membrane.

Mandibular Bar: First of the five pharyngeal arches and is interposed between the mouth and the first ectodermal (visceral or pharyngeal) groove. The major portion, or mandibular process, of this arch forms the lower jaw.

Meatus: An opening or passage; general term for an opening or passageway in the body.

Mesoderm: In early stages, the mesoderm is known as the mesoblast. The mesoderm is the middle of the three primary germ layers of the embryo, lying between the ectoderm and the endoderm. It gives rise to the connective tissue, bone, cartilage, muscle, blood and blood vessels among others.

Morula: 16 celled embryo. Derived from the Latin for Mulberry.


Nasofrontal: Pertaining to the nasal and frontal bones.

Neuropore: The open anterior end or the open posterior end of the neural tube of the early embryo. These openings gradually close as the tube develops, the timing of each being so precise that they are used to define horizons XI and XII.

Notochord: The rod-shaped body, comprised of cells from the mesoblast, below the primitive groove of the embryo, defining the primitive axis of the body.


Occipital: The back part of the head, located near the occipital bone.

Olfactory: Pertaining to olfaction, or the sense of smell.

Olfactory Pit: The primordium of a nasal cavity. Each pit was created by the sinking of an olfactory placode amd os bounded by a medial and lateral nasal process.

Olfactory Placode: An oval area of thickened ectoderm on either ventrolateral surface surface of the head or the early embryo, constituting the first indication of the olfactory organ.


Pharyngeal: Arches give rise to numerous derivatives such as skeletal and muscular structures of the face, eears, pharynx and larynx. In lower vertebrates known as the branchial arches.

Plexus: A network or tangle: general term for a network of lymphatic vessels, nerves, or veins.

Polar Body: The small abortive cells with a haploid chromosome complement, consisting of a tiny piece of cytoplasm and a nucleus, resulting from an unequal division of the primary oocyte.

Primitive Node: A mass of cells at the cranial end of the primitive streak, related to the organization of the embryo.

Primitive Streak: A faint white trace at the caudal end of the embryonic disc, formed by the movement of cells at the beginning of the mesoderm formation. It provides the earliest indication of the embryonic axis.

Pronuclei: The haploid nucleus of a fully mature ovum which loses its nuclear envelope and liberates its chromosomes to meet in the synapses with those similarly derived from the male pronucleus.


Sclera: The tough white outer coat of the eyeball, covering approximately the posterior five sixths of its surface, and continuous anteriorly with the cornea and posteriorly with the external sheath of the optic nerve.

Segmentation Cavity: The fluid-filled cavity of the mass of cells produced by cleavage of a fertilized ovum.

Somite: One of the paired, block like masses of mesoderm, arranged segmentally alongside the neural tube of the embryo, forming the vertebral column and segmental musculature.

Spinal Ganglia: The craniospinal ganglia on the dorsal roots of the spinal nerves.

Sulci: A groove, trench or furrow.

Syncytiotrophoblast: The outer syncytial layer of the trophoblast, eventually gives rise to outer layer of the placenta and other extra embryonic structures.


Temporal: Pertaining to the lateral region of the head, above the zygomatic arch.

Tragus: The cartiliginous projection anterior to the external opening of the ear.

Trophoblast: A granular protoplasmic body; a plastid, formed from cytotrophoblast, eventually surrounds the entire embryo; insert themselves between uterine epithelial cells, then spread along the epithilial surface of the basal lamina that underlies the endometrial epithelium to form a trophoplate.


Ventralward: Toward a belly, venter, or ventral aspect.

Vertex: The top or crown of the head.


Yolk Sac: The yolk sac in man contains no nutrients so in this aspect is vestigial. However, the its embryonic endodermal roof is the source of the mucous membrane of almost the entire intestinal tract. The yolk sac also is concerned with the transfer of nutritive fluid to the embryo from the trophoblast, primary mesenchyme and extra-embryonic coelem.


Zona Pellucida: A thick, transparent, noncellular layer or envelope of uniform thickness surrounding an oocyte. Also called the oolemma.