THE STRUCTURE OF THE CHROMOSOMES 



135 



Beginning with the anaphase, it is observed that each chromosome 

 consists of matrix and chromonema. The latter is double, i.e., there are 

 in reality two chromonemata; these may lie very closely parallel to each 

 other and so appear as a single thread, or they may be rather widely 

 separated. The chromaticity of the matrix usually obscures this struc- 

 ture unless special methods are employed. At the end of the anaphase 

 the chromosomes tend to form a fairly compact group at each pole 

 (tassement polaire), a condition which may be accentuated by fixation. 

 The chromonemata become somewhat more contorted when the chro- 

 mosomes shorten. 



As the telophase begins, the chromaticity of the matrix decreases 

 markedly, so that the chromonemata are more easily observed. As each 

 group of chromosomes enlarges and becomes a telophase nucleus, the 



Fi 



early 

 proph 

 and 8 



G. 74. — The chromonemata in somatic mitosis in Harmanthus. 1, anaphase; 2, 

 telophase; S, interphase; 4< early prophase; 6, prophase; G, mid-prophase; 7, late 

 ase; 8, metaphase. The magnification in S, 5, 6, and 7 is greater than that in 1,2, 4, 

 {After Telezynski, 19316.) 



chromonemata of the several chromosomes become joined together by 

 anastomoses to form the reticulum. The origin and the nature of these 

 connecting strands are uncertain. They have generally been regarded as 

 short lateral extensions of some component of the chromonema, but in 

 Zea microsporocytes they are clearly formed by the matrix (McClintock) 

 (Fig. 65a). As the telophasic changes continue, the chromonemata of 

 the various chromosomes become increasingly difficult to distinguish 

 as individuals. In rapidly multiplying nuclei where the interphase is 

 short, the next prophase may begin before they are lost to view; otherwise 

 the condition of the interphase passes into that characteristic of the 

 metabolic stage, when the reticulum is finer and relatively uniform in 

 texture. Such fineness of texture is probably due in some measure to a 

 tendency on the part of the two crooked chromonemata in each chro- 

 mosome to become more independent during this stage. 



Little is known about the fate of the matrix in the telophase and the 

 origin of the achromatic constituents of the nucleus. Because of the 

 large volume of the karyolymph in which the reticulum is imbedded at 

 the end of the telophase, it is obvious that at least some of its material 



