CHROMOSOME STRUCTURE 181 



Throughout the mitotic cycle the more chromatic constituent 

 of the chromosomes persists in the form of two chromonemata. 

 The less chromatic constituent forms a matrix in which the chromo- 

 nemata lie for the major portion of the cycle. It was found im- 

 possible to obtain any information as to whether it remains distinct 

 from the matrix material of the other chromosomes and the karyo- 

 lymph at interphase or whether it merely loses its identity between 

 nuclear divisions. 



At anaphase the chromosome is made up of an achromatic 

 matrix and a more chromatic constituent ; the latter has the 

 structure of a zig-zag thread or chromonema. This thread fre- 

 quently appears double, i.e., there are two chromonemata present 

 which may either remain close together at the upper region of the 

 chromosome or separate rather widely at the lower portion. With 

 the close of anaphase, the chromosomes shorten, and at the incep- 

 tion of telophase, the chromaticity of the chromosome matrix 

 decreases considerably, leaving the chromonemata more clearly 

 visible. At telophase the chromonemata are more contorted than 

 at anaphase, owing to the shortening of the chromosome as a whole. 

 Mutual connections now make their appearance by means of 

 anastomoses with neighbouring chromosomes to form the reti- 

 culum of the daughter nuclei. 



The alterations observable in the chromosomes at telophase are 

 continued into interphase, when the individual chromonemata 

 can still be observed. Finally, the anastomoses of the main portion 

 of the chromonemata become more and more alike, so that the 

 latter are no longer discernible in the meshwork of the resting 

 nucleus. In the meantime the matrix material of several chromo- 

 somes becomes entirely achromatic and apparently continuous. At 

 early prophase the reticulum is resolved into separate tracts, 

 which represent the chromosomes in which the chromonemata 

 become more and more visible with the disappearance of the 

 anastomoses. The matrix of each chromosome also becomes more 

 visible than the surrounding karyolymph. The chromonemata 

 now become more clearly distinct and regular, giving the aspect 

 characteristic of the spiral stage. At the middle of prophase the 

 chromonemata straighten out, thicken and diverge a little as they 



