| germinal membrane | In many eggs with a large amount of yolk, cell division (cleavage) is restricted to a superficial layer of the fertilized egg (meroblastic cleavage). This layer is termed the blastoderm. In birds it is a flat disc of cells at one pole of the egg and in insects an outer layer of cells surrounding the yolk mass. (18 Nov 1997) |
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| germinal mosaicism | Gonadal mosaicism, a state in which cells in a sector of a gonad are of a form not present in either parent, because of mutation in an intermediate progenitor of that sector. (05 Mar 2000) |
| germinal mutation | A mutation in the germ cells (the cells which will undergo meiosis to form the gametes). Such mutations are therefore passed on to offspring. (09 Oct 1997) |
| germinal pole | <cell biology> In most animal oocytes the nucleus is not centrally placed and its position can be used to define two poles. That nearest to the nucleus is the animal pole and the other is the vegetal pole, with the animal vegetal axis between the poles passing through the nucleus. During meiosis of the oocyte the polar bodies are expelled at animal pole. In many eggs there is also a graded distribution of substances along this axis, with pigment granules often concentrated in the animal half and yolk, where present, largely in the vegetal half. (12 Nov 1997) |
| germinal rod | One of the minute elongated bodies resulting from the repeated division of the oocyst during sporogony. In the case of the malarial parasite, it is the form that is concentrated in the salivary glands and introduced into the blood by the bite of a mosquito; it enters the liver cells (exoerythrocytic cycle), whose progeny, the merozoites, infect the red blood cells to initiate clinical malaria. Synonym: germinal rod, zoite, zygotoblast. Origin: sporo-+ G. Zoon, animal (05 Mar 2000) |
| germinal 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) |
| germinal vesicle | The nucleus of a primary oocyte, the development of which is suspended in prophase I of the first meiotic division between embryohood and sexual maturity. (09 Oct 1997) |
| active centre | The part of a macromolecule at which a substrate or ligand, upon binding, produces biological activity; for an enzyme, this is the catalytic centre, the site on an enzyme that catalyses the reaction. (05 Mar 2000) |
| anospinal centre | The centre in the spinal cord that controls the contraction of the anal sphincter. (05 Mar 2000) |
| asymmetric centre | <chemistry> Same as a chiral centre, a tetrahedral carbon atom that bears four different substituents. (09 Jan 1998) |
| Broca's centre | The posterior part of the inferior frontal gyrus of the left or dominant hemisphere, corresponding approximately to Brodmann's area 44; Broca identified this region as an essential component of the motor mechanisms governing articulated speech. Synonym: Broca's area, Broca's field, motor speech centre. (05 Mar 2000) |
| Budge's centre | The preganglionic motor neurons in the first thoracic segment of the spinal cord which give rise to the sympathetic innervation of the dilator muscle of the eye's pupil. Synonym: Budge's centre. (05 Mar 2000) |
| vasomotor centre | Diffuse area of the reticular formation in the lateral medulla containing neurons that control vascular tone; consists of separate vasodepressor and vasopressor areas. (05 Mar 2000) |
| reaction centre | <plant biology> The site in the chloroplast that receives the energy trapped by chlorophyll and accessory pigments and initiates the electron transfer process. A photosynthetic complex containing chlorophyll (or bacteriochlorophyll) and other components, within which occurs the initial electron transfer reactions of photophosphorylation. (09 Oct 1997) |
| catalytic centre | See: active centre. (05 Mar 2000) |