100 



FUNDAMENTALS OF CYTOLOGY 



surely regrettable that other groups of organisms do not have giant 

 chromosomes. 



Conclusions. — The chromosome should be thought of as a persistent 

 individual that reproduces only by division and in this sense maintains 

 its individuality throughout successive nuclear and life cycles. In every 

 nuclear cycle it passes through a series of alterations which may obscure 

 its continuity although they do not disprove it. There is no evidence that 

 individualized masses of matrix persist, but all the evidence obtained 

 directly and indirectly {e.g., by X-ray alterations induced during the 

 metabolic stage) indicates that the chromonema with its characteristic 

 longitudinal differentiation in structure and function does persist. The 





Iff 



Fig. 74. — Chromosomes of Drosophila melanogaster. a, prophase in nucleus of giant 

 cell of ganglion of female. Proximal ends of X-chromosomes separated from distal por- 

 tions by nucleolar material; kinetochores marked 1. b, chromosomes at metaphase of 

 mitosis in ganglion cell of male, c, X- and F-chromosomes associated with nucleolus 

 during prophase. (After B. P. Kaufmann.) 



chromonemata in the metabolic stage are relatively free from enveloping 

 matrix and lie more exposed to the other nuclear materials, which indi- 

 cates that the chromosomes exert their effects upon cell activities mainly 

 at this time. Their compact arrangement within a matrix appears to be 

 significant in connection with mitotic distribution rather than with 

 metabolism. Later on we shall point out how alterations occur in the 

 organization of the chromonema from time to time, its parts being 

 rearranged or exchanged with those of other chromonemata, but these 

 changes are not such as to invalidate the basic concept of chromosomal 

 individuality and continuity. 



The various nuclear materials or ultimate units necessar}- to the 

 normal life of the organism are nearly everywhere carried in several 

 chromosomes rather than in only one or a very large number. This 

 small group, or genome, is to be regarded as an organized system of 

 interdependent members, and not as a simple collection of materials. 

 Studies on altered chromosomes show that the primary requisite is the 

 presence of the right assortment of vmits or materials and that their 



