GLOSSARY 581 



in regard to particular parts of chromosomes ; possible in some hybrids 



and polyploids; hence " equational exceptions." D., 1929 c ; Haldane, 



1930 a. 



Gametic , meiosis immediately before fertilisation. 



Zygotic , meiosis immediately after fertilisation. 



Reduction Division, formerly applied to the one of the meiotic divisions at 



which a particular author thought reduction and segregation occurred. 

 Reductional Division, a separation of homologous parts of chromosomes 



derived from opposite parents at anaphase of a first or second division 



(as opposed to equational). 

 Reductional Split, the same at diplotene. Wenrich, 1916. 

 Restitution Nucleus, a single nucleus formed through failure the first 



division. Rosenberg, 1927. 

 Rings, (i) at mitosis, chromosomes with no ends. 



(ii) at meiosis, chromosomes associated end to end in a ring, usually 



by terminal chiasmata ; especially applied to diploid interchange 



heterozygotes where more than two chromosomes are so associated. 

 [Satellite], a trabant, q.v. 

 Segment, a portion of a chromosome which can conveniently be considered 



as a unit for a given purpose ; e.g., Differential segment. Cf. Belling, 



1927 h. 

 Segregation, the separation of chromosomes of paternal and maternal origin at 



meiosis and the separation of the differences observed genetically. Bateson, 



1902. 



Secondary , the segregation in an allopolyploid of differences 



between its ultimate diploid parents. D., 1928. 



Effective , that which gives viable gametic or zygotic combinations 



(especially following multiple association in structural hybrids). D., 



1931- 

 [Semi-heterotypic Division], a first division which gives rise to a restitution 



nucleus following defective pairing. Rosenberg, 1927. 

 Set of Chromosomes, a minimum complement of chromosomes derived from 



the gametic complement of a supposed ancestor. 

 Sex-Chromosome, one which in the heterozygous sex is mated with a dissimilar 



homologue ; the X and Y chromosomes, q.v. Wilson, 1906. 

 Sexual Differentiation, the production by an organism or by related organisms 

 of gametes of two sizes such that the larger can fuse only with the smaller. 

 Sexual Reproduction, that which requires meiosis and fertilisation. 

 Sperm, Spermatozoon, the male gamete in animals. 

 Spermatocyte, sperm mother-cell. 



Spindle, the axially differentiated part of the cytoplasm within which the 

 centromeres of the chromosomes are held during metaphase and anaphase. 

 Normally bipolar, exceptionally unipolar or multipolar. 

 [Spindle Attachment], the position of the centromere. 



Spiral, a coil of the chromosome thread (chromosome or chromatid), at 

 mitosis or meiosis. 



Internal , a coil within a single chromatid between prophase and 



anaphase. 



Relational , coiling of two chromatids or chromosomes round one 



another. 



Major , the larger internal coil at meiosis. 



Minor , the smaller internal coil. 



Relic , the coiling which survives at telophase and prophase. 



Super , larger coils derived in prophase from the rearrangement of 



relic spirals. 



