SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 167 



la I Icr. The case for discontinuity is strongest in the absence of any chromosome 

 order, and becomes progressively weaker with the establishment of definiteness 

 and precision in form and behavior." 



"So far as I can see there is no half way ground between the assumption that 

 the chromosomes are definite, self-perpetuating organic structures and the other 

 which presents them as mere incidental products of cellular action. According 

 to one view individual chromosomes are descendents of like elements and possess 

 certain qualities and behavior because of their material descent, the visible 

 mechanism for which is the process of mitosis : according to the other any similari- 

 ties that may exist in the complexes are the result of chance aggregations of non- 

 specific materials. It is a choice between organization and non-organization in 

 the last analysis, at least in terms of cellular structures. To attempt the sub- 

 stitution of a conception of molecular organization, which is beyond the ex- 

 perience of the biologist and which exceeds the present powers of the chemist to 

 analyse, is to cast aside all hope of solving the problem of cellular action, because 

 it is necessary to understand, not only the physical and chemical phenomena 

 involved, but also their different forms in the various parts of the cell." 



"That the chromosomes do not maintain a compact and easily recognizable 

 form in the interval between mitoses is accepted by many . . . biologists as proof 

 that they no longer exist as entities. All the other manifold indications of char- 

 acter and continuity do not weigh against this apparent loss of identity. Doubt- 

 less it would be more satisfying if we could at all times perceive the chromosomes 

 in unchanging form in all stages of cellular activity, but why we should demand 

 this condition as a test of individuality in the chromosomes when we unhesitat- 

 ingly admit the unity of the organism in all the varied changes of its develop- 

 ment from a single cell, through such complexities of change and metamorphosis 

 as to give rise to doubts of even the phyletic position of some stages, it is difficult 

 to see. Being organic, the chromosomes must change their form, they must suffer 

 division of their substance and they are obliged to restore this loss through meta- 

 bolic changes. Since these changes of substance take place at surface contacts 

 there is an obvious advantage in increased superficies and, in common with other, 

 larger structural elements, the chromosomes become extended and their sub- 

 stances are diffused. In this state their boundaries may not be well defined and 

 this circumstance has been seized upon as a disproof of their continuity." 



"Since it is not possible to observe directly the action of the chromosome we 

 are obliged to make use of indirect evidence, seeking parallels between elements 

 of structure and action in the chromosomes, and the mass effect of cellular action 

 as exhibited in the so-called body characters. Such a method is justified by all 

 other experience in tracing relations between structure and function in oTganisms, 

 and while it apparently resolves the organism into parts of greater or less in- 

 dependence, has given us -our best conceptions of it as a whole." 



"What is postulated ... is that the chromosomes are self-perpetuating 

 entities with individual peculiarities of form and function to identify them. 

 Characteristics of form and behavior we see; certain very definite parallels be- 

 tween these and the manifestations of somatic characters exist beyond question; 

 provision for the perpetuation of the organic unity of the individual chromosomes 

 is found in the process of mitosis; the actual direct result of its operation appears 

 in the uniform conditions of the complex in the individual animal; the extension 



