90 



FUNDAMENTALS OF CYTOLOGY 



and conditions. Furthermore, it is not known to what degree the 

 threads may be longitudinally compound below the range in which X rays 

 can affect their parts individually. It may therefore be helpful to dis- 

 tinguish provisionall,y three levels of doubling: (1) elementary doubling, 

 in which the ultimate longitudinal constituents (protein chains?) of 

 the chromosome become duplicated or multiplied, probably through 

 the formation of new ones close to the old ones b}^ a process analogous to 

 crystallization or polymerization ; (2) effective doubling, in which the thread 

 somehow reaches a stage at which a given agency such as X rays may 

 affect one longitudinal fraction and not another; (3) visible doubling, in 

 which a thread appearing single under the microscope becomes double 

 by a process that looks like a real splitting. 



Studies on chromosome structure are complicated by the fact that 

 the chromonemata are spirally coiled in some degree at all stages of 



^ hi 



Anaphase Telophase Early Prophase Lafe Prophase Metaphase 



Fig. 62. — Diagram of the chronionema coiling cycle through mitosis. While the gyres 

 (a) of one series are relaxing and disappearing, a new series (b) is developed; thus two coiling 

 cycles overlap in the mitotic cycle. Further explanation hi text. (Based on mitosis in 

 Trillium m.icrospore as described by A. H. Sparrmv.) 



the somatic nuclear cj^cle. During this cycle the changes undergone 

 appear to be somewhat as follows (Figs. 02, 63). In anaphase the two 

 spirally coiled chromonemata recently formed by division in the prophase 

 (page 61) lie mostly close together and twisted about each other. In 

 the enlarging telophase nucleus they tend to separate somewhat and 

 relax their coils, although the number of spiral turns, or gyres, remains 

 about the same as it was in anaphase. In the following early prophase 

 these gyres begin to disappear, but before the uncoiling is completed 

 each of the constituent chromonemata (chromatids) begins independently 

 to form numerous new^ gyres. In the advancing prophase these new 

 gyres become fewer and larger while the chromatids gradually' untwist. 

 By the end of the prophase the chromatids have lost their twists and old 

 gyres (relic coils) and have developed individual matrices. At this 

 time the chromonemata, with their new gyres now closer together, reveal 

 the doubleness which becomes effective in the next mitotic cycle. The 

 metaphase chromosome thus consists of two chromatids which have 

 become nearly or completely untwisted and in each of wliich there are two 



