112 THE CHROMOSOMES 



Polyaomy : A condition in which one or more chromosonies, 

 but not the entire set are present in the polyploid state. 



Pycnoais : A condition in which all the chromosomes of a 

 nucleus have fused together to form a single mass — 

 occurs only in moribund cells. 



Quadrivalent : A multivalent composed of four chromosomes. 



Ring Chromosome : A chromosome in which the two ends have 

 fused together so that it forms a continuous circle. 



Rotation {of chiaamata) : The relative rotation of the four 

 ' arms ' of a bivalent on either side of a chiasma, which 

 often occurs between early diplotene and diakinesis. 



Salivary Oland Chromosomes : Chromosomes in the nuclei 

 of the salivary gland cells in Diptera. These chromo- 

 somes have undergone complete somatic pairing ; 

 consequently what is ordinarily called a saUvary 

 gland chromosome is thus really two chromosomes fused 

 side by side. 



Spindle Attachment : A special region of the chromosome by 

 which the rest of the chromosome is attached to the 

 spindle at metaphase and anaphase. The spindle 

 attachment is a special ' organ ' of the chromosome 

 which can be seen at all stages of mitosis and meiosis, 

 under favourable conditions. It does not divide at the 

 same time as the rest of the chromosome. 



Spindle Elements : The elements of which the spindle is prob- 

 ably composed and which usually correspond in number 

 to the chromosomes. {Central spindle element : a 

 spindle element which is not related to any of the 

 chromosomes and which forms the * core ' of the spindle 

 surrounded by the other elements which lie parallel to it. 



Spindle Fibres : Fibres which were supposed to exist in the 

 substance of the spindle, running from pole to pole or 

 from pole to equator. 



Stem Body : The equatorial part of the spindle which elongates 

 at anaphase and telophase, forming a long strand between 

 the two resulting nuclei. 



Synchronous Mitosis : The occurrence of a number of cell 

 divisions which take place at exactly the same time in 

 a group of neighbouring cells. 



Terminalization : Shifting of chia^mata from their original 

 positions towards the end of the bivalent. 



Tetrad : See Bivalent. 



Trivalent : A multivalent composed of three chromosomes. 



Univalent : A chromosome which has not undergone pairing 

 at the zygotene stage or one in which pairing has come 

 to an end due to failure of chiasma-formation. 



Zygotene : The stage of meiosis which follows on leptotene and 

 precedes pachytene. The stage when pairing of the 

 homologous ohromoisomes takes place. 



