| germ layer | <embryology> A layer of cells produced during the process of gastrulation during the early development of the animal embryo, which is distinct from other such layers of cells, as an early step of cell differentiation. The three types of germ layers are the endoderm, ectoderm, and mesoderm. Diploblastic organisms (e.g. Coelenterates) have two layers, ectoderm and endoderm, triploblastic organisms (all higher animal groups) have mesoderm between these two layers. Germ layers become distinguishable during late blastula/early gastrula stages of embryogenesis and each gives rise to a characteristic set of tissues, the ectoderm to external epithelia and to the nervous system for example: although some tissues contain elements derived from two layers. (18 Nov 1997) |
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| parietal layer | The outer layer of an enveloping sac or bursa, usually lining the walls of the cavity or space occupied by the enveloped structure, the structure itself being covered with the inner or visceral layer of the enveloping sac; an actual or potential space is enclosed by the two layers and intervenes between parietal and visceral layers. The parietal layer is usually the more substantial layer. Synonym: lamina parietalis. (05 Mar 2000) |
| germ layer theory | The developmental biology theory that during early development, the animal embryo divides itself into two or three germ layers, each of which then proceed to further differentiate into organs and tissues specific to that particular layer. (09 Oct 1997) |
| parietal layer of leptomeninges | Like a cobweb, covered with or consisting of soft fibres or hairs so entangled as to give a cobwebby appearance. (09 Oct 1997) |
| parietal layer of serous pericardium | The outer part of the serous pericardium suported by the fibrous pericardium. Synonym: lamina parietalis pericardii. (05 Mar 2000) |
| parietal layer of tunica vaginalis | The outer part of the tunica vaginalis testis supported by the internal spermatic fascia. Synonym: lamina parietalis tunicae vaginalis testis. (05 Mar 2000) |
| visceral layer | The inner layer of an enveloping sac or bursa which lines the outer surface of the enveloped structure, as opposed to the parietal layer which lines the walls of the occupied space or cavity. The visceral layer is usually thin, delicate and not apparent as being separate, but rather appears to be the outer surface of the structure itself. Synonym: lamina visceralis. (05 Mar 2000) |
| visceral layer of serous pericardium | The inner part of the serous pericardium applied directly on the heart. Synonym: epicardium, lamina visceralis pericardii. (05 Mar 2000) |
| visceral layer of tunica vaginalis of testis | The inner part of the tunica vaginalis testis applied directly to the testis and epididymis. Synonym: lamina visceralis tunicae vaginalis testis. (05 Mar 2000) |
| medial cartilaginous layer | The broad medial portion of the cartilaginous part of the auditory tube. Synonym: lamina medialis cartilaginis tubae auditivae, lamina cartilaginis medialis tubae auditivae, medial cartilaginous layer, medial layer of cartilaginous auditory tube. (05 Mar 2000) |
| medial layer of cartilaginous auditory tube | The broad medial portion of the cartilaginous part of the auditory tube. Synonym: lamina medialis cartilaginis tubae auditivae, lamina cartilaginis medialis tubae auditivae, medial cartilaginous layer, medial layer of cartilaginous auditory tube. (05 Mar 2000) |
| vitelline layer of egg | <zoology> The membrane, usually of protein fibres, immediately outside the plasmalemma of the ovum and the earlier stages of the developing embryo. Its structure and composition vary in differing animal groups. (18 Nov 1997) |
| reticular layer of corium | The thicker deep layer of the corium consisting of dense irregularly arranged connective tissue. Synonym: reticular layer of corium, stratum reticulare cutis, tunica propria corii. (05 Mar 2000) |
| cerebral layer of retina | The internal layer of the retina containing the neural elements, as distinguished from the outer leaf of the retina, or pigmented layer. Synonym: pars optica retinae, neural layer of retina, optic part of retina, stratum cerebrale retinae. (05 Mar 2000) |
| membranous layer | The connective tissue membrane that, with the lateral lamina, completes the lateral and inferior walls of the cartilaginous part of the auditory tube. Synonym: lamina membranacea cartilaginis tubae auditivae, membranous layer. (05 Mar 2000) |