CHAPTER X 

 THE STRUCTURE OF THE CHROMOSOMES 



In most fixed and stained preparations the chromosomes in the 

 metaphase and anaphase of mitosis appear as uniformly colored, more 

 or less homogeneous bodies. Attention must now be given to the 

 important question of the actual internal structure of such chromosomes 

 and the alterations undergone by this structure during the various phases 

 of the mitotic cycle. Students of this subject have investigated inten- 

 sively somatic chromosomes, particularly those of root tips, but sporo- 

 cytes and spermatocytes have come to be the favorite objects for such 

 studies because of the size of their nuclei and the ease with which they 

 may be examined without imbedding or sectioning. 



In the development of our knowledge of chromosome structure three 

 principal interpretations have been prominent. These are (1) the 

 chromonema theory, according to which the fundamental structural 

 element of the chromosome is a slender filament, or chromonema, present 

 in some form throughout the mitotic cycle and associated with a second 

 substance, the matrix; this interpretation was used as a basis for our 

 preliminary description of mitosis in Chapter VIII; (2) the alveolation 

 theory, according to which the filament is a temporary form assumed by 

 the chromatic nuclear matter in the prophase as the result of an alveola- 

 tion of the more or less homogeneous chromosome in the preceding 

 telophase; (3) the chromomere theory, according to which small autono- 

 mous bodies, the chromomeres, are supported by relatively achromatic 

 threads and reproduce regularly by division in mitosis. It will be seen 

 in what follows that our present interpretation of chromosome structure 

 incorporates elements of the chromonema and chromomere theories; 

 that is, the typical chromosome in its condensed stages is now regarded 

 as a mass of matrix enclosing one or more chromonemata in which there 

 are small differentiations called "chromomeres." 



The Chromonema Theory. — In 1880 Baranetzky examined fresh 

 Tradescantia sporocytes in spring water and saw what appeared to be 

 spiral filaments within the chromosomes. Janssens (1901) suggested 

 that such spirals in the telophase were the same as those observed by 

 himself and many others in the prophase. This conception was embodied 

 in the chromonema hypothesis, developed especially by Bonnevie 

 (1908, 1911) and Vejdovsky (1912). As a result of her studies on 

 Ascaris, Allium, and other forms, Bonnevie concluded that each chromo- 



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