CHAPTER VII 

 THE CHROMOSOMES 



It is not difficult to account for the fact tfiat thie chromosomes have 

 long held a major share of the attention of cytologists. They are indi- 

 vidualized protoplasmic units present in definite numbers and multiplying 

 regularly and only by division. Because of the precision with which 

 they are distributed at mitosis, every nucleus of the developed plant 

 or animal body has a descendant of every chromosome present when 

 development w^as initiated. Furthermore, when reproduction occurs 

 the chromosomes are passed on to the next generation through the spores 

 or gametes. Their physicochemical composition is such that they have 

 specific and profound effects upon the course of development and hence 

 upon the organism's characters. As a consequence of all this, they play 

 a major role in heredity. 



Some of the above points were brought out in the chapter on cell 

 division. It was also showTi there that the chromosome has two main 

 structural constituents: the chromonemata, present throughout the 

 entire nuclear cycle, and the matrix, which is conspicuous only during 

 certain phases of mitosis. The chromosome, although it is a persistent 

 individual reproducing by division in every mitotic cj'cle, passes in each 

 cycle through a series of structural changes that alter its appearance 

 profoundly'. At metaphase and anaphase it is clearly evident as a dis- 

 tinct individual, whereas in the metabolic stage, when its chief functions 

 are being performed, it is rarely possilile to determine its limits. 



In this chapter we shall consider in greater detail the form and the 

 structure of chromosomes, chiefly in somatic tissues, and discuss the 

 constitution of the chromosome complement, or outfit of chromosomes 

 making up a given nucleus. 



Somatic Chromosomes. — The general morphology of somatic chromo- 

 somes is best displayed at the metaphase and anaphase of mitosis. If 

 special technical methods are used, much can also be learned about their 

 structure at these stages, but so far the most reliable information of this 

 kind has come from chromosomes passing through meiosis. Chromo- 

 somes may dilTer greatly in size in different organisms, in unlike tissues, 

 and in some degree in plants grown in different nutrient solutions. 

 Fixation often affects their size. Nearly all chromosomes at anaphase 

 lie between 1 and 20fi in length. It is easy to understand why investiga- 



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