SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 157 



every chromosome present in the first cell of the series. The great theoretical 

 importance of these facts will be apparent when we take up the subject 

 of chromosome reduction, and the application of cytological phenomena 

 to the problems of heredity. 



THE INDIVIDUALITY OF THE CHROMOSOME 



In later chapters the question of the significance of the nuclear struc- 

 tures in heredity is to be considered. In connection with this question 

 it is of the highest importance to determine whether or not the chromo- 

 somes to which the reticulum gives rise in the prophase are in any real 

 sense the same as those which went to make up the reticulum at the pre- 

 ceding telophase. That they do preserve their identity as individuals 

 through the resting stage, arise only by division, and maintain therefore 

 a genetic continuity throughout the life cycle, was held by van Beneden 

 (1883), Rabl (1885) and Boveri (1887, 1888, 1891) many years ago, and 

 since that time the idea has received the support of a large number of 

 investigators. We shall now briefly review some of the evidences which 

 have led the majority of cytologists to the view that the chromosomes, 

 "if . . . not actually persistent individuals, as Rabl and Boveri have 

 maintained, . . . must at least be regarded as genetic homologues that 

 are connected by some definite bond of individual continuity from gen- 

 eration to generation of cells" (Wilson 1909). 



The Frequent Persistence of Visible Chromosome Limits in the 

 Resting Reticulum. In the foregoing description of the behavior of the 

 chromosomes in mitosis it was pointed out that in rapidly dividing tissue 

 the telophasic alveolation of the chromosomes and their anastomosis to 

 form the reticulum often do not proceed far enough during the interphase 

 to obliterate the boundaries between the chromosomes, which separate 

 again in the ensuing prophase without having lost their visible identity. 

 In such nuclei there can be little doubt that the autonomy of the chromo- 

 some is preserved. In other cases, however, the telophasic transforma- 

 tion of the chromosomes is more complete and the resulting reticulum 

 reacts very weakly to the stains, so that the limits of the constituent 

 chromosomes disappear from view completely. Many workers have 

 therefore objected to the statement that here also the chromosomes are 

 present as individuals, although invisible. Haecker (1902) and Boveri 

 (1904) pointed out that this objection may be met by assuming that it is 

 the achromatic framework of the alveolized chromosome, and not neces- 

 sarily the basichromatic fluid held within it, that maintains a structural 

 independence. This view had the support of the earlier observation 

 made by Boveri (1887o, 1888a, 1891; also 1909) and confirmed by Herla 

 (1893), that the chromosomes in the segmenting egg of Ascaris have a 

 certain arrangement when they build up the nuclear reticulum in the 

 telophase and reappear from the reticulum in the same position at the 

 next prophase. 



