SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 151 



which, however, the limits of the component chromosomes can be 

 distinguished until a very late stage. 



Interphase (Fig. 51, /). It often happens that in rapidly growing 

 tissue, such as the meristem of the root tip, the mitoses succeed one 

 another so rapidly that the telophasic changes may not proceed far 

 enough to obscure the limits of the chromosomes in the reticulum 

 before the changes of the ensuing prophase begin. In such tissue it is 

 not always possible to tell whether a given nucleus will undergo further 

 telophasic change or will at once enter upon the prophases. Such 

 interphasic nuclei develop nucleoli, but karyosomes (in species which 

 have these bodies) are usually not formed until a more advanced stage. 



Resting Stage (Fig. 51, J, K). In slowly growing tissue the successive 

 mitoses do not follow one another with very great rapidity, and the 

 telophasic changes are carried on until the condition characteristic of 

 the typical resting nucleus is reached: the interphase here becomes the 

 prolonged resting stage. The structure of the resting nucleus has been 

 fully described in Chapter IV. In the reticulum the limits of the con- 

 stituent chromosomes usually become indistinguishable, although it is 

 known that in certain cases such nuclei, if properly sectioned and stained, 

 may reveal heavier and lighter areas in the reticulum which represent 

 respectively the chromosomes and the regions of anastomosis between 

 them. The importance of these facts will be apparent in our treatment 

 of the individuality of the chromosomes. 



Prophase (Fig. 51, L-Fig. 52). The first indication that the prophasic 

 changes have begun is seen in the breaking down of the reticulum in 

 certain regions. In the case of nuclei which show heavier and lighter 

 areas in their reticula this breaking down occurs along the light portions. 

 In view of what has been said concerning the origin of the reticulum at 

 telophase it is apparent that the breaking up of the reticulum in the 

 prophase represents in such cases the separation of the constituent 

 chromosomes from each other along the lines of their telophasic union, 

 and it has been inferred that a similar interpretation applies to those 

 nuclei in which the reticulum is perfectly uniform or in which the nuclear 

 material assumes more irregular forms. In this way there are developed 

 from the resting reticulum a number of more or less distinct reticulate 

 units, which, in view of their subsequent behavior, we know to be the 

 chromosomes (Fig. 51, L, M). That these units are essentially the same 

 as those which went to make up the reticulum at the preceding telophase 

 seems highly probable; there can be little doubt on this point when the 

 interphase is short. 



The material of each reticulate unit (chromosome) now gradually con- 

 denses in a very irregular fashion about its open spaces and cavities. 

 The thinner regions bounding these spaces and cavities become broken 

 down, and the thicker portions remain as a very irregular zigzag thread of 



