GLOSSARY 583 



Trabant (German), a segment of a chromosome, separated from the rest by 

 one long constriction if terminal or two if intercalary ; usually so short 

 that longitudinal contraction reduces it to a sphere before it attains the 

 characteristic chromatid diameter. 



Intercalary , one with full-sized chromosome limbs on both sides 



of it. 



Lateral , where the trabant is a branch of the chromosome. 



Translocation, change in position of a segment of a chromosome to another 

 part of the same chromosome or ol a different chromosome. Bridges, 

 1917. 



Triploid, an organism having three sets of chromosomes. Nemec, 19 10. 



Trisomic, an organism with three chromosomes of one type ; usually other- 

 wise diploid {2X -\- i). A diploid organism is said to be doubly trisomic, 

 not tetrasomic, when it has the two extra chromosomes of two different 

 types (2;v + I + i). 



Secondary , a trisomic organism in which the extra chromosome 



has two identical ends. Belling, 1927 6. 



Tertiary , a trisomic organism in which the extra chromosome is 



made up of halves corresponding with the halves of different chromo- 

 somes in the normal set. Belling, 1927 b. 



Trivalent {v. Univalent). 



Univalent, a body at the first meiotic division corresponding with a single 

 chromosome in the complement; especially when unpaired. Bivalent, 

 Trivalent, Quadrivalent, Quinquevalent, Sexivalent, Septivalent, Octa- 

 valent (for Octovalent), etc., are associations of chromosomes held 

 together between diplotene and metaphase of the first division by 

 chiasmata. Similar associations of more than two including non- 

 homologous chromosomes in structural hybrids should be described as 

 " unequal bivalents," " associations " or " rings " of three, four, etc. 

 The hybrid forms, " monovalent," " tetravalent," ' pentavalent," 

 " hexavalent," etc., used in chemistry should be avoided. 



[W-chromosome], sometimes used for the X-chromosome where the female 

 is the heterozygous sex, Z being then used for Y. 



X-chromosome, with diploid sex differentiation, the sex chromosome in 

 regard to which one sex is homozygous ; this is said to be the homozygous 

 sex. With sex differentiation in the haploid, the sex chromosome of the 

 female. 



Y-chromosome, the sex chromosome that is present and pairs with the X in 

 the sex heterozygote. 



Zygote, the cell formed by the union of gametes and the individual derived 

 from it (c/. Homozygote). 



Zygotene, the pairing threads (and the stage at which they occur) in the 

 prophase of meiosis. v. Winiwarter, 1900. 



