14 



Genetics, Cells, and Chromosomes 



no structure is generally visible inside the metaphase or ana- 

 phase chromosome. If, however, the cells are pretreated before 

 fixation with weak ammonia, hot water, or several other agents, 

 and stained with crystal violet, each chromatid is seen to consist 

 of a thin, coiled thread, the chromonema (Fig. 5), or gene string, 

 surrounded by a wide matrix. 



Although some visual evidence seems to indicate that the 

 chromonemata become double during late prophase or early meta- 



FiG. 5. Coiled chromonemata in Trillium, (a) Anaphase, (b) Late 

 diakinesis showing five pairs of chromosomes, each with its four chromatids, 

 (c) Diakinesis, showing two unpaired chromosomes; two pairs of chromo- 

 somes are at the top and one pair is at the right side; at the bottom are 

 the paired E chromosomes, each of whose arms is as long as most of the 

 other pairs. (Photomicrographs courtesy of Dr. C. L. Huskins.) 



phase one cell division before that at which the halves separate 

 to opposite poles, the chromosomes, when subjected to bombard- 

 ment by X-rays, usually behave as a single structure at meta- 

 phase, anaphase, and telophase and in the resting stage and do 

 not behave as a double structure until early prophase or just 

 before prophase. Apparently the reason for this is that at meta- 

 phase and anaphase, the chromosome as a w^hole is single even 

 though the chromonema inside may be double, and it is the 

 whole chromosome that reacts to the X-rays. 



The process by which a new chromonema forms from an old 

 one is not adequately known, but there is some evidence that 

 each constituent part of a chromonema, the gene, regenerates 

 another identical with and alongside itself. These new genes 

 then become joined up, and a duplicate of the original chro- 

 monema is formed. 



