THE STRUCTURE OF THE CHROMOSOMES 143 



length in the earher portion of the prophase. ^^ At this stage all the 

 threads taken together have a total length of about 1,500/i, and as the 

 prophase advances they become reduced to about one-tenth of this 

 length. This brings the ''ultimate chromomeres " into contact and causes 

 coiling of the chromatic thread so formed. Aggregates of these chromo- 

 meres, together with additional chromatic matter, constitute the con- 

 spicuous "compound," or " secondary, " chromomeres seen in the later 

 portion of the prophase. 



Although the chromosomal threads have chromomeres and probably 

 always shorten in some measure during the prophase of mitosis in sporo- 

 cytes, the appearance of the chromosome at the end of the prophase 

 varies greatly in different plants. In Tradescantia and Secale rather 

 uniform chromonemata lie coiled within the matrix (Fig. 77), but in Zea 

 the large chromatic accumulations in the threads continue to be visible, 

 the chromosome showing many of the characteristic structures of the 

 early prophase in spite of its greatly reduced length (Figs. 66, 152). 

 Hence such chromosomes, like those of grasshoppers (Fig. 79), are 

 especially well suited to studies on the functions of their component 

 parts. Vegetative nuclei {e.g., in root tips) commonly show rather 

 smooth chromonemata from mid-prophase onward, though it is likely 

 that improved methods will reveal more concerning the architecture of 

 such threads, at least in their earlier stages. 



A useful hypothesis covering the above observations is that the 

 chromatic substance appearing in small masses, or chromomeres, at 

 certain regions of the thread, together with any additional chromatic 

 matter, varies in amount at different stages of the nuclear cycle and at 

 a given stage in different organisms. When such chromatic substance 

 is scanty the chromomeres are distinct, but when it is abundant they are 

 more or less united and the thread appears more uniform. Hence the 

 chromomeres should be expected to appear most plainly in the early 

 prophase, when the masses of chromatic substance are small and well 

 separated from one another because of the extended and straightened 

 condition of the thread. 



That the structural differentiation exhibited by the chromonemata 

 determines in large part the general form of the metaphase chromosome 

 seems evident. In Zea, for example, the general pattern formed by 

 the spindle-attachment region, the nucleolus-forming region, larger 

 chromomeres, and chromatic "knobs" in the early prophase is maintained 

 as the chromosome shortens, but the positions of the smaller elements 

 become difficult to make out (Fig. 66). It remains to be ascertained to 



" Early pachynema stage. The threads are paired in synapsis, hence the chromo- 

 meres appear double. In the late pachynema stage each component thread shows 

 its split, the chromomeres then appearing as groups of four "chromioles" (see Chapter 

 XVI). 



