the cell division process. Regardless of diflferences in their detailed 

 morphology, both direct and indirect evidence makes it clear that each 

 chromosome is an individual entity which, barring accidents, is retained 

 throughout the life of the cell. Earlier ideas suggesting that chromosomes 

 arose during interphase as the result of coalescence of individual particles 

 or that they were joined together to form a continuous chain or spireme, 

 have had to be discarded. The electron microscope has not, so far, 

 provided any good basis for detailed description of the submicroscopic 

 structure of the chromosome either at interphase or any other stage. The 

 best pictures to date indicate that the individual strands (chromone- 

 mata) of the chromosome are made up of many microfibrils which 

 measure about 60 A units. The total number of these microfibrils is the 

 subject of some controversy, although it seems likely that it is upwards 

 of 64 in number. Indirect evidence suggests that each chromosome at 

 interphase is made up of at least two bundles of microfibrils. As already 

 noted, parts of some chromosomes appear as highly condensed areas in 

 the interphase nucleus which are known as heterochromatic segments 

 (prochromosomes, chromocenters) and are usually thought to represent 

 tightly coiled chromosome regions. 



The detailed morphology of the chromosomes is best studied during 

 cell division at the stages of greatest contraction, such as metaphase and 

 anaphase. By the usual methods of fixation and staining the chromo- 

 somes at these stages appear as somewhat cylindrical bodies constricted 

 at one or more places along their length (Figure 4-12 (a) and (b) ). The 

 positions of these constrictions are a constant characteristic of any par- 

 ticular chromosome of the complement. In the older literature they were 

 frequently referred to as primary and secondary constrictions; the former 

 representing a specialized region of the chromosome which plays a major 

 role in movement of the chromosomes during division, while the latter 

 occur only in certain chromosomes — being associated in some, but not 

 necessarily all, cases with the production of nucleoli (nucleolar zone). 

 The primary constriction, which is known by many terms, the commonest 

 of which are "centromere" and "kinetochore," is obviously an essential 

 part since chromosomes which lack it fail to orient or move properly 

 during mitosis. 



With appropriate treatment the chromosome may be seen to consist 

 of one or more helically coiled threads apparently embedded in a matrix 

 (Figure 4-12). The coiled threads are called the chromonemata (singu- 

 lar, chromonema) and are considered to represent the permanent part 

 of the chromosome. The reality of the matrix has been challenged by 



88 / CHAPTER 4 



