THE CHROMOSOMES ' 85 



tors have preferred larger ones, like those of amphibians and liliaceous 

 plants, for studies of chromosomal constitution. Special mention will 

 be made of the giant salivary-gland chromosomes of certain insect 

 tissues later in the chapter. 



The form and the structure of a typical somatic anaphase chromosome 

 are represented in Fig. 57. It is an elongate body consisting of matrix 

 and two spiral chromonemata recentl}^ formed by division. The two 

 may be so closely associated that they seem to be one, or they may 

 appear as clearly separate threads more or less twisted about one another. 

 They represent the chromatids which will separate in the anaphase of 



Fig. 57. — A typical somatic chromo- 

 some at anaphase of mitosis. Semi- 

 diagrammatic, c, kinetochoie; ch, chro- 

 monema; h, heterochromatic region; m, 

 matrix; s, satellite; no, nucleolus organizer. 





■\ 



'^^■^ 



\ 



\ 



\f« 



y 



/ 



Fig. 58. — Chromosome G in meiotic 

 prophase in maize, showing its nucleolus 

 organizer {no) in contact with the large 

 nucleolus; short region to right of it is the 

 satellite, c, kinetochore. 



the next mitotic cycle. Along the chromonemata are small lumps, the 

 chromomeres. These are rarel}^ evident in preparations of anaphase 

 chromosomes even when the matrix has been rendered transparent, but 

 during the prophase, when the threads have less chromatic material, 

 they frequently show clearl3\ They are best studied in meiotic pro- 

 phases (Figs. 58, 64, 6; 78). 



An important feature of the chromosome is the specialized region 

 at which its reactions with the spindle mechanism seem to be largely 

 centered. This region is called the kinetochore. (Other terms in the 

 literature are centromere, primary constriction, and kinomere.) The kine- 

 tochore commonly appears as a relatively achromatic region. In certain 

 large chromosomes it can be seen to be traversed by two slender strands 

 which evidently represent the chromonemata. In the double metaphase 

 chromosome there are accordingly four (Fig. 59). In some instances a 

 minute body, the kinosome, has been made out at the middle of each 



