GLOSSARY 579 



[Monoploidj , " True Haploid," q.v. 



Monosomic, a diploid organism lacking one chromosome of its proper 

 complement (v. Trisomic, Tetrasomic). 



Mosaic, in animals, the equivalent of a chimaera, q.v. In plants, a chimaera 

 produced by repeated mutation. 



Mother-cell, the cell with a diploid nucleus which by meiosis gives four 

 haploid nuclei, e.g.. the spore mother-cell in Bryophyta and Pteridophyta, 

 the microspore or pollen mother-cell and the megaspore or embryo- 

 sac mother-cell in phanerogams. The sperm mother-cell is known 

 as the spermatocyte, the egg mother-cell as the oocyte, in animals. 

 Many other terms, used in the lower plants, are unnecessary in general 

 cytology. 



Multivalent [v. Univalent). 



[Non-cellular], unicellular organisms in the Protista. Dobell. 



Non-disjunction, cytologically, the failure of separation of paired chromo- 

 somes at meiosis and their passage to the same pole ; genetically, any 

 results that might be imputed to such an abnormality, although usually 

 arising from the failure of pairing, or from multivalent formation. 

 Bridges, 1914. 



Non-homologous pairing, association of non-homologous parts of chromosomes 

 at pachytene ; cf. torsion-pairing. McClintock, 1933 ; cf. D., 1935 c. 



Nuclear sap, the fluid which is lost by the chromosomes as they contract 

 during prophase and which fills the space of the nucleus. 



Nucleolar Organiser, specific chromomere responsible for developing the 

 nucleolus. McClintock, 1934. 



Nucleolus, a body in the nucleus which disappears and does not resolve itself 

 into chromosomes at mitosis. 



Nucleus, a cell body reproducing by mitosis. The most constant constituent 

 of animal and plant cells. Robert Brown, 1831. 



[Oligopyrene] , of spermatozoa, those deficient in chromosomes. 



Oocyte, egg mother-cell. 



Orientation, the movement of chromosomes so that their centromeres lie 

 axially with respect to the spindle, either as to their potential halves 

 at mitosis [auto-orientation) or as to members of a pair in meiosis [co- 

 orientation). 



[Orthoploid], polyploid organisms with a number of balanced, complete, 

 chromosome sets. Cf. Aneuploid. (Originally used for those with any 

 even number of chromosomes, v. Winkler, 1916.) 



Pachytene, the double thread (and the stage at which it occurs) produced by 

 pairing of the chromosomes in the prophase of meiosis, v. Winiwarter, 

 1900. 



Pairing of Chromosomes, (active) the coming together of chromosomes at 

 zygotene or (passive) the continuance of their association at the first 

 metaphase of meiosis. 



Somatic , the lying of homologous chromosomes especially close 



to one another at metaphase of mitosis. 



Secondary , the same phenomenon as seen amongst bivalents at 



meiosis. D., 1928. 



Parasynapsis, the association of chromosomes side by side observed at zygo- 

 tene, as opposed to their alleged end-to-end association at this stage, 

 or " telosynapsis." Wilson, 191 2. 



Parthenogenesis, a form of apomixis in which the female gamete develops 

 without fertilisation. Owen, 1849. 



• Diploid , that in which meiosis has failed first. 



Haploid , that in which fertilisation fails first. 



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