THE STRUCTURE OF THE CHROMOSOMES 137 



telophasic karyolymph comes in contact with the cytoplasm. According 

 to the interpretation of the metaboHc nucleus as a group of chromosomes 

 in contact, the membrane may be regarded as being made up of the 

 surfaces of the outermost members of the group. In the prophase it is 

 often observed that the formed chromosomes remain in contact with the 

 nuclear boundary over a portion of their surface, particularly at their 

 spindle-attachment regions. At this stage the membrane obviously must 

 extend from these areas of contact over the mass of prophasic karyo- 

 lymph, or separated spindle substance. This point requires further 

 study. 



As the prophase begins, the individual chromonemata composing the 

 reticulum separate from one another through a disappearance of the 

 anastomoses. It seems probable that the substance of the latter becomes 

 a part of the matrix appearing about the chromonemata. At first the 

 chromonemata stand out as contorted double threads, but they soon 

 tend to straighten out. From this stage onward each chromosome, 

 composed of a matrix with the halves of the split chromonema along its 

 sides, undergoes a progressive thickening and often some shortening. 

 Meanwhile the matrix becomes more abundant and highly chromatic, 

 obscuring the contorted chromonemata within it. Toward the end of the 

 prophase the matrix undergoes division, so that the chromosome as a 

 whole, as well as its chromonema, is double. 



The prophase now passes into the metaphase, each chromosome estab- 

 lishing connection with the spindle figure by its spindle-attachment 

 region. In the large chromosomes here being considered each longitu- 

 dinal half of the chromosome shows its chromonema to be double.^ 

 Exactly when this subdivision of the chromonema takes place is not 

 known. Certain appearances suggest that it first becomes visible some 

 time during the prophase when the chromonemata are still more or less 

 extended, although the multiplication of elements within the thread may 

 well occur at some other stage of the mitotic cycle. The new chromo- 

 nemata so formed remain together through the ensuing anaphase and 

 separate in the anaphase of the next mitosis. In other words, the 

 chromonema divides one mitotic cycle in advance of the anaphase in 

 which the halves so formed are to separate. 



Slender and Small Chromosomes. — To what extent the above 

 interpretation of chromosome structure and the nuclear cycle can be 

 applied to chromosomes of small dimensions is not known. It has been 

 suggested that, if the division of the nuclear elements is related in some 

 definite way to their growth and absolute mass, the stage at which the 

 chromonema becomes visibly double may not be the same for all nuclei. 

 On the other hand, small chromosomes may differ from larger ones only 

 in the size or the compactness in arrangement of their constituent ele- 

 9 Kaufmann (1926a), Sharp (1929), Nebel (1932), Hedayetullah (1931.) 



