| primitive perivisceral cavity | The space between the ectoderm and endoderm in the gastrula. Synonym: primitive perivisceral cavity. (05 Mar 2000) |
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| primitive pit | The depression in the primitive node that serves to connect the notochordal canal with the surface ectoderm and the yolk sac. These connections are referred to as the neurenteric canal. (05 Mar 2000) |
| primitive reticular cell | A cell with processes making contact with those of other similar cell's to form a cellular network; along with the network of reticular fibres, the reticular cell's form the stroma of bone marrow and lymphatic tissues. (05 Mar 2000) |
| primitive ridge | One of the paired ridge's on either side of the primitive groove. (05 Mar 2000) |
| primitive streak | An ectodermal ridge in the midline at the caudal end of the embryonic disk from which arises the intraembryonic mesoderm; achieved by inward and then lateral migration of cells; in human embryos, it appears on day 15 and gives a cephalocaudal axis to the developing embryo. Synonym: germinal streak. (05 Mar 2000) |
| neuroectodermal tumour, primitive | A malignant brain tumour sharing common features and biologic properties with medulloblastoma. Some pathologists and clinicians use the terms interchangeably: they both can disseminate throughout the nervous system and, in some cases, systemically. most lesions arise from the posterior fossa in children under five years of age. The tumour often produces hydrocephalus and symptoms of increased intracranial pressure. Papilledema is often present. (12 Dec 1998) |
| polar | Describes a feature or phenomenon occuring at the end (pole) of an object (such as a planet) or organism (such as an amoeba). <chemistry> Describes a molecular that has a permanent electric dipole. See: polar group. Compare: nonpolar, nonpolar groups. (12 Mar 1998) |
| polar amino acid | An alpha-amino acid in which the functional group attached to the alpha-carbon (i.e., R in RCH(NH2)COOH) has hydrophilic properties; e.g., serine, cysteine, homocysteine. (05 Mar 2000) |
| polar anaemia | A form of anaemia sometimes observed in natives of temperate climates when they migrate to the Arctic or Antarctic regions. (05 Mar 2000) |
| polar body | <cell biology> In animals each meiotic division of the oocyte leads to the formation of one large cell (the egg) and a small polar body as the other cell. Polar body formation is a consequence of the very eccentric position of the nucleus and the spindle. (18 Nov 1997) |
| polar cataract | A capsular cataract limited to an area of the anterior or posterior pole of the lens. (05 Mar 2000) |
| polar cell | <cell biology> In animals each meiotic division of the oocyte leads to the formation of one large cell (the egg) and a small polar body as the other cell. Polar body formation is a consequence of the very eccentric position of the nucleus and the spindle. (18 Nov 1997) |
| polar compound | A compound in which the electric charge is not symmetrically distributed, so that there is a separation of charge or partial charge and formation of definite positive and negative poles; e.g., H2O. See also inorganic compound. (05 Mar 2000) |
| polar fibres | Those fibres of the mitotic spindle extending from the two poles of the spindle toward the equator. Compare: astral fibres, kinetochore fibres. (05 Mar 2000) |
| polar flagellation | <cell biology> Condition of having flagella attached at one end or both ends of the cell. (09 Oct 1997) |