TIII'J CHROMOSOMES 



87 



the number ot" chromosoine.s in a metabolic nucleus by counting such 

 heterochromatic masses (euchromocenters) (Fig. 16). Other such regions 

 may occur elsewhere in the chromosome, the nucleolus organizer fi-e- 

 quentl}^ having this character. Structurally, such portions seem to be 

 regions in which the chromonemata are more closely coiled, and by 

 treatment vnih. NH4CI it has been found possible to relax the coils. 



The significance of heteropyknosis is not yet fully evident. Recent 

 researches oji the giant chromosomes in the salivary glands of the fruit 

 fly {Drosoyhila) indicate some connection between heterochromatic 

 regions and the nucleic acid cycle. It is thought to have a role in the 

 synthesis of the thymonucleic acid in the nucleus and also to affect the 

 ribonucleic acid content of the cytoplasm of the egg. Since the nucleolus 

 also contains ribonucleic acid compoimds, it, too, appears to be involved 



'W I ^ 



Fig. 59. — Portion of somatic chromo- 

 some of Trillium at metaphase, showing 

 structure of kinetochore. Description in 

 text. {AflrrL. W. Sharp.) 



Fig. 60. — Two cells recently formed by 

 division of one in root tip of onion. Chro- 

 mocenters at opposite poles of the pair of 

 nuclei. 



in this chemical cycle. This is stronglv emphasized by the origin of the 

 nucleolus in direct connection with the nucleolus organizer, which is 

 typicall}' heterochromatic, and by the reciprocal relationship existing 

 between the chromosomal and nucleolar cycles: the nucleolus appears 

 as the telophase chromosomes gradually lose their stainability, and it 

 disappears as the prophase chromosomes regain it. This cy(;lic change 

 has alwaj'S puzzled cytologists, and now it seems that a solution of the 

 puzzle is being fovmd. 



In the chapter on cell division, reference was made to an uncertainty 

 regarding the number of chromonemata actually present in a chromo- 

 some. Some observers do not admit the presence of more than one at 

 anaphase and telophase, many hold that there are two, and some believe 

 that there are four, each possibly having its own individual matrix. 

 Since visual observation involves the interpretation of stmctures so near 

 the limit of visibility, more refined techniques have been brought to 

 bear upon the problem. The most promising of these has been irradia- 



