CHAPTER IX 

 THE MORPHOLOGY OF THE CHROMOSOMES 



As they pass through a mitotic cycle, the chromosomes undergo a 

 remarkable series of changes in form and structure. Nevertheless, 

 in the metaphase and anaphase, when they are most condensed and 

 clearly evident as individuals, they exhibit many characteristics of 

 size and form which have proved useful not only in determining the 

 functions with which they are associated, but also in comparing related 

 species and hybrids. For the study of chromosome morphology in 

 vascular plants the root tips and dividing microspores are particularly 

 favorable. In animals the tissues of young embryos and the sperma- 

 togonia have been most frequently used. Different fixing reagents 

 may produce a considerable variation in the appearance of chromosomes, 

 but the studies of S. Nawaschin, Lewitsky, Taylor, and others have 

 furnished us with methods which are very trustworthy when properly 

 employed.^ 



Size and General Form. — As they appear in the metaphase or 

 anaphase of somatic mitosis, the chromosomes of different organisms 

 show a great range in size. In some cases they are extraordinarily 

 minute, being less than Iju long, while in others they may reach a length 

 of 20iu or more; the breadth may vary likewise. Some natural groups, 

 such as fungi and certain insect orders, have small chromosomes as a 

 rule, while in others, notably amphibia, grasshoppers, and liliaceous 

 plants, they are characteristically large. Within narrow circles of 

 affinity, chromosome size sometimes affords evidence for probable 

 taxonomic relationship. In this connection it should be noted that the 

 size may be somewhat altered by cultural conditions and microtechnical 

 treatments. 



The chromosome is typically an elongated body. Sometimes very 

 small chromosomes may appear practically spherical at metaphase or 

 anaphase, but they are usually elongated in the prophase. Moreover, 

 studies on larger chromosomes lead to the inference that even the smallest 

 spherical chromosome contains an elongated chromonema. The essential 

 point is that every chromosome adequately known possesses a longi- 

 tudinally differentiated organization. This differentiation belongs 

 primarily to the chromonema, but it is usually evident also in the entire 

 metaphase chromosome with its abundant matrix. 



' For an account of the development and present status of our knowledge of 

 chromosome morphology, see Lewitsky (1931a); see also 19316 for fixation. 



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