GLOSSARY 575 



Configuration, an association of chromosomes at meiosis, segregating in- 

 dependently of other associations at anaphase. 



Congression, the movement of chromosomes on to the metaphase plate. 



Conjugation, the pairing of the chromosomes, gametes or zygotes, or the fusion 

 of pairs of nuclei. 



Constriction, an unspiralised segment of fixed position in the metaphase 

 chromosome. Agar, 191 1. 



Primary or centric , that always associated with the centromere. 



Nucleolar , a secondary constriction determined by the organisa- 

 tion of the nucleolus. 



Secondary , any acentric constriction. D., 1926. 



Co-orientation, the relative orientation of two centromeres on the spindle. 



Crossing-over, the exchange of corresponding segments between corresponding 

 chromatids of different chromosomes by breakage and reunion following 

 pairing ; a process inferred genetically from the reassociation of linked 

 factors in mendelian hybrids and cytologically from the reassociation 

 of parts of chromosomes in structural hybrids. Morgan, 191 1. 



Effective , that which is detectable in breeding experiments. 



Illegitimate , crossing-over in a haploid or polyploid which is not 



a structural hybrid, between homologous and reduplicated segments of 

 two chromosomes which being structurally dissimilar as a whole, do not 

 normally pair. Determines secondary structural change. D., 1932. 



Double , the production of a chromosome in which crossing-over 



has occurred twice ; may be reciprocal or non-reciprocal as between 

 chromatids. Sturtevant, 1914. 



[Cytokinesis], the separation of daughter-cells, usually after nuclear division. 



Cytoplasm, the protoplasm apart from the nucleus. 



Daughter chromosome {v. Chromatid). 



Deficiency, loss of a terminal acentric segment of a chromosome from the 

 diploid complement. Bridges, 191 7. 



Deletion, loss of an intercalary acentric segment of a chromosome. 



Diakinesis, the last stage in the prophase of meiosis — immediately before the 

 disappearance of the nuclear membrane. Haecker, 1897. 



Dicentric, of a chromatid or chromosome, having two centromeres. 



Differential Affinity, the failure of two chromosomes to pair at metaphase in 

 the presence of a third although they pair in its absence. D., 1928. 



Differential Segment, one in respect of which two pairing chromosomes differ 

 in a permanent hybrid (by contrast with a pairing segment). D., 1931 c, 

 1934 a. 



Diminution, the loss or expulsion of a part of the chromosome complement at 

 mitosis so that a daughter nucleus is formed lacking this part. 



Diplochromosome, chromosome which has divided once more often than it 

 normally does in relation to its centromere (c/. attached X in Drosophila 

 melanogaster) . White, 1935. 



Diploid, (i) the zygotic number of chromosomes [zn) as opposed to the gametic 

 or haploid number {n) ; (ii) an organism having two sets of chromosomes 

 {;2x) as opposed to organisms having one (haploid) three (triploid) or 

 more sets (x, ^x, etc.) (v. Polyploid). Strasburger, 1905- 



Functional , allopolyploid which behaves as a diploid in segre- 

 gation. D., 1928. 



Diploidisation, process of division of conjugate nuclei in fungi, by which 

 haploid cells or mycelia become diploid. B idler, 1930. 



[Diplont], organism at the diploid stage of the life-cycle, as opposed to 

 " haplont." 



Diplophase, [(i)] the diplotene stage of meiosis, Belling, 1928 ; (ii) the diploid 



