| centrolecithal | Denoting an ovum in which the deutoplasm accumulates centrally. Origin: centro-+ G. Lekithos, yolk (05 Mar 2000) |
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| centrolecithal egg | An egg in which the yolk is concentrated near the centre of the egg cell, as is the case in many of the insects. (05 Mar 2000) |
| centrolecithal ovum | One in which the yolk is mostly located near the centre of the egg, as in arthropods. (05 Mar 2000) |
| segmentation | <biology> Organisation of the body into repeating units called segments is a common feature of several phyla, for example arthropods and annelids, although the segments arise by very different mechanisms. Segmentation also occurs during embryonic development in vertebrates, for example partition of the mesoderm into somites and is a feature of early CNS development. See: rhombomeres, neuromeres. (18 Nov 1997) |
| segmentation cavity | The cavity in the blastula of a developing embryo. Synonym: blastocoele, cleavage cavity, segmentation cavity. Origin: blasto-+ G. Koilos, hollow (05 Mar 2000) |
| segmentation gene | <molecular biology> Genes required for the establishment of segmentation in the embryo. In Drosophila about 20 such genes are required. A development pattern of the embryo of the fruit fly Drosophila where the embryo looks like a stack of discs. Each disc is a segment, and each segment consists of two compartments (an anterior and posterior). Various parts of the adult fruit fly (wings, legs, eyes, etc.) arise from specific segments. (09 Oct 1997) |
| segmentation nucleus | The compound nucleus in the impregnated ovum, formed by conjugation of the nuclei of the ovum and spermatozoon (female and male pronuclei), the zygote nucleus after it commences the first cleavage division. (05 Mar 2000) |