THE STRUCTURE OF THE CHROMOSOMES 147 



and the growth period in animal oocytes, the basichromatic character of 

 the chromonema largely or completely disappears and the limits of the 

 individual matrix, whether this is temporarily swollen or removed, 

 become indistinguishable. Later on, a reverse series of changes takes 

 place and the chromosome is again evident as an individual. This is not 

 to be construed as an actual destruction and recreation of the chromosome 

 but rather as a cyclic change undergone by a definite group of materials 

 which must assume a structure suitable at one stage to the performance of 

 metabolic functions and at another stage to individual division and 

 distribution to daughter nuclei. 



The above conclusion seems obvious enough when, as has been 

 frequently observed, a single chromosome separated from its neighbors 

 forms an individual metabolic nucleus and later passes through an 

 essentially normal series of division stages. The formation of a mitoti- 

 cally dividing nucleus by a single isolated chromosome, taken together 

 with what is known of karyomeres and merokinesis, serves to emphasize 

 the compound nature of the ordinary nucleus and leads to the view that 

 the usual mitotic figure is essentially a group of associated chromosomes 

 undergoing division and separation in unison. That the same inter- 

 pretation is to be placed upon the chromosome when it is a member of a 

 group, whether as a karyomere (Fig. 75) or an indistinguishable portion 

 of a metabolic nucleus, is now required by the evidence. That something 

 essential in the characteristic structural and functional organization of 

 each chromosome persists through the metabolic as well as other stages 

 of the cycle is practically proved in recent investigations on the effects of 

 X-ray treatments (Chapter XVIII). The results of treatments during 

 the metabolic stage indicate that even at this time the peculiar linear 

 organization of certain elements of the chromosome is normally main- 

 tained. In other words, the genes are somehow held to their character- 

 istic linear order at all times. No other plausible explanation of the 

 facts has been suggested. Only our limited vision prevents us from seeing 

 directly that a given chromosome has as characteristic an organization 

 in the metabolic stage as it has in other stages. This being the case, 

 less direct but none the less cogent evidence must be relied upon. What 

 persists as a chromosome through the nuclear cycle is evidently a group 

 of substances held together in a characteristic pattern (in a chromonema 

 or "chromonema axis") and accompanied at certain periods by additional 

 materials, the whole system undergoing a recurrent cycle of physico- 

 chemical transformations in successive nuclear generations. To what 

 extent the achromatic materials remain individualized is not known. 



The chromosome, therefore, stands out as a persistent individual 

 reproducing by division. It does not follow, however, that this individ- 

 ual necessarily remains rigidly fixed and unaltered in character in succes- 

 sive generations. It will be shown in subsequent chapters that a portion 



