110 THE CHROMOSOMES 



Chromatid : A longitudinal half of a chromosome in prophase 

 or metaphase ; the two chromatids of each chromosome 

 separate from each other at the anaphase of mitosis, 

 so that a telophase chromosome consists of one chromatid 

 only. 

 Chromomere : A granule on a prophase chromosome at mitosis 

 or meiosis, or on a salivary gland chromosome. 

 Chromomeres are now believed to be identical with genes. 

 Chromosome : At prophase and metaphase of mitosis, two 

 chromatids and a spindle attachment ; at meiosis half 

 a bivalent. 



{Branched Chromosome : a chromosome in which the 

 chromatids fork dichotomously, either at the spindle 

 attachment or elsewhere.) 



{Ring Chromosome : a chromosome in which the two 

 ends are fused together, in such a way that a con- 

 tinuous circle results.) 

 Diakinesis : The last part of the prophase of the first meiotic 

 division, between diplotene and prometaphase. See 

 Fig. 9e. 

 Differential Segment : A segment of a chromosome which is 

 not present in another chromosome that is otherwise 

 homologous. The opposite to pairing segment. 

 Diploid Set of Chromosomes : A group of chromosomes which 

 can be divided into two equal haploid groups. 



{Diploid organism : an organism which has a diploid 

 set of chromosomes in each of its somatic cells.) 

 Diplotene : The stage in the prophase of the first meiotic 

 division which follows on pachytene and precedes 

 diakinesis. See Fig. 9d. 

 Fixation : The process of killing and coagulating a cell by 

 means of some chemical or physical agency. 



{Fixable : a nucleus which can be killed and coagu- 

 lated without seriously altering its visible morphology ; 

 unfixable : a nucleus which is seriously altered in 

 structure by the process of fixation.) 

 Heterogametic : Producing gametes of more than one kind, 

 which differ as to the chromosomes which they contaiii. 

 The opposite to homogametic. 

 Heteropycnosis : the property of contracting or condensing at 

 a different rate from the majority of the chromosomes in 

 the nucleus. 



{Negative heteropycnosis : condensing more slowly 

 than the other chromosomes do.) 



{Positive heteropycnosis : condensing faster, earlier 

 or more completely than the other chromosomes.) 

 Homogametic : Producing gametes which are all alike as to 

 the chromosomes which they contain. The opposite 

 to heterogametic. 



