THE STRUCTURE OF THE CHROMOSOMES 145 



On the basis of this hypothesis — and it is at present no more than a 

 hypothesis — the chromomere is regarded not as a fundamental autono- 

 mous unit but rather as a product of such a unit, the gene. As already 

 suggested, a way may be found between the view that the chromomere is 

 a fundamental unit and the view that it is only a chance thickening of 

 the thread, if we construe it as a mass of material definitely associated 

 with some localized reaction and often becoming continuous with neigh- 

 boring masses. The hypothesis summarized above goes a step further and 

 attributes these localized reactions to genes. Similarly, the division of 

 the chromonema is thought to result from the division of the genes. At 

 present very little can be said concerning the time relation which may 

 exist between gene-division (or other mode of multiplication) and the 

 visible splitting of the chromonema. It is conceivable that a double 

 chromonema might be more highly compound as regards its gene strings. 

 Finally, it should be pointed out that although the chromosome may have 

 a characteristic lengthwise differentiation visibly manifested by its 

 chromomeres, as well as a functional differentiation manifested in the 

 results of genie activity, there is as yet insufficient evidence to war- 

 rant a confident statement regarding the spatial and functional relation- 

 ships of chromomeres and genes. Research in progress promises to 

 yield evidence on this important point (see, further, p. 318). 



The Continuity of the Chromosome. — In each succeeding prophase in 

 the nuclei of a growing tissue there appears a group of chromosomes made 

 up of a certain number of characteristically different individuals. Are 

 these chromosomes in any real sense the same as those which developed 

 the nucleus in the preceding telophase? That they do preserve their 

 identity as individuals through the metabolic stage, arise only by division, 

 and therefore maintain a genetic continuity throughout the life cycle, was 

 held by many early observers. ^^ The question has been much debated, 

 and, although many statements of the conception of chromosomes as 

 persistent entities have been too crude, it remains true that the chromo- 

 somes "must at least be regarded as genetic homologues that are con- 

 nected by some definite bond of individual continuity from generation 

 to generation of cells" (Wilson, 1909c). 



The evidences upon which this view is based are briefly as follows. 

 With rare exceptions, which have been found not to militate against any 

 proper conception of continuity, the chromosomes in successive mitoses 

 are the same in number and form. When the total number is altered in 

 any way, or when a number of the group is changed by loss, fragmenta- 

 tion, translocation, or the like, the altered group appears in the next 

 mitosis; the original complement is in no way restored during the inter- 

 vening metabolic stage. The limits of the several chromosomes remain 

 visible through this stage in certain nuclei ; in extreme cases the nucleus is 



20 E.g., van Beneden (1883), Rabl (1885), and Boveri (1887, 18886, 1891). 



