86 ' FUNDAMENTALS OF CYTOLOGY 



strand. The portions of the chromosome on either side of the kineto- 

 chore are known as arms. These are equal or unequal in length depending 

 upon the kinetochore's position, which is constant for a given chromo- 

 some. Telokinetic chromosomes, i.e., those with terminal kinetochores, 

 have been reported in animals, but they seem to be very rare in plants. 

 A number of supposedly telokinetic chromosomes have been shown to 

 have a minute second arm. The present tendency is to regard the 

 telokinetic condition, at least in plants, as an abnormality that does not 

 long persist. In metaphase and especially in anaphase the chromosome 

 tends to be bent at the kinetochore. 



Chromosomes may have more or less prominent "secondary con- 

 strictions" in one or both of their arms. Special methods may reveal 

 more of these than appear after ordinary treatments. Commonly one 

 chromosome of each of the genomes, or basic outfits composing a nucleus, 

 has in one arm an especially prominent secondary constriction with 

 which there is associated a particular function, the organization of the 

 nucleolus. The small segment of the chromosome distal to this con- 

 striction is called a satellite. In the anaphase the exact extent of the 

 nucleolus organizer, or specialized region directly concerned in the develop- 

 ment of the nucleolus, is not evident, for the chromosome is very compact 

 and no nucleolus is present. Its features appear much more clearly in 

 certain plants during the meiotic prophase, when the chromosome is 

 extended and devoid of matrix (Figs. 58; 64, a). During the telophase, 

 as the matrix loses its stainability and disappears, the nucleolus makes 

 its appearance in connection with the chromonemata at or near the 

 constriction. Hence the number and the position of the nucleoli in 

 the resulting metabolic nucleus are dependent upon the number and 

 location of the nucleolus-forming chromosomes in the telophase. It is 

 known that the material for the nucleoli is derived from all the chromo- 

 somes present, but in some manner it is collected or organized as a 

 nucleolus only at the nucleolus organizer. In the ensuing prophase the 

 nucleolus commonly disappears, partially or completely, as the matrix 

 accumulates and becomes highly stainable. 



Another important feature of chromosomal constitution is heteropyk- 

 nosis. This term, which means "difference in density," refers to the 

 condition present when all or a definite part of a chromosome remains 

 denser and more highly chromatic than the other chromosomes or parts 

 through the nuclear cycle. In the anaphase the stained chromosome 

 may exhibit this feature weakly or not at all, but during the telophase 

 the heterockromatic part retains its compactness and stainability while 

 the euchromatic parts undergo the usual telophasic transformation (Figs. 

 60; 64, d). Most commonly it is the regions near the kinetochore that 

 are heteropyknotic. In some plants this makes it possible to estimate 



