CYTOLOGY 23 



nucleus there is always one pair which show a marked constriction near one 

 end, on which no matrix accumulates. The small terminal portion, beyond 

 the constriction, is thus isolated from the main body of the chromosome 

 and is termed a satellite, while the chromosomes themselves are known as 

 the sat-chromosomes.* This constriction is associated with the formation 

 of the nucleolus and is called the nucleolar organizer. The nucleolus 

 makes its appearance attached to the base of this constriction, which is 

 usually heterochromatic (see below), and as the nucleolus enlarges the 

 satellites may adhere to its surface. The nucleolar material is apparently 

 secreted by the sat-chromosomes during telophase, while during prophase 

 it is dissipated. It is either adsorbed on to the chromosomal surface as a 

 pellicle, or, according to another view, it is wholly or in part dispersed into 

 the cytoplasm. As we have already mentioned, it contains ribo-nucleic acid, 

 which is found in cytoplasm, especially in the mitochondria. 



There is at least one pair of sat-chromosomes in a diploid nucleus and 

 each of them forms a nucleolus. If they happen to lie close together at 

 telophase the two nucleoli may coalesce into one, otherwise they remain 

 apart. The presence of more than two nucleoli in a nucleus is usually a sign 

 that more than the diploid complement of chromosomes is present, that is 

 to say that the nucleus is polyploid. 



The matrix of the metaphase chromosomes is known to consist of thymo- 

 nucleic acid, the molecules of which align themselves in rows, parallel to 

 the protamine molecules of the chromonemata. Underlying the other 

 changes during mitosis there is thus a cycle of the charging and discharging 

 of nucleic acid upon the chromonemata. Certain parts of a chromonema 

 may, however, retain their charge throughout the metabolic phase ; these 

 portions remaining deeply stained in telophase and even in the metabolic 

 nucleus, where they may be seen as knots or chromocentres in the reticulum. 

 This retained material is called heterochromatin, and the rest is dis- 

 tinguished as euchromatin. A chromonema which carries heterochromatin 

 is called heteropycnotic. Charges of heterochromatin occur most commonly 

 near the kinetochore and near the nucleolar organizer in sat-chromosomes. 

 The heterochromatin appears to be responsible for the formation of the 

 thymo-nucleic acid of the euchromatin in the rest of the matrix and possibly 

 also for the ribo-nucleic acid of the nucleolus, since the latter always appears 

 in contact with a heterochromatic region. 



The chromonema itself is not uniform throughout its length, but shows 

 a beaded structure, in which the beads or chromomeres stain more deeply 

 than the intervening portions. These chromomeres are the centres of attach- 

 ment of the matrix substance. They are generally regarded as the seats of 

 the genes or units of heredity, which we shall speak about under Genetics. 



The chromosomes which are formed in mitosis have always a constant 



and characteristic number in every true species of plant and animal (see table 



on p. 33), and they are also constant in form. Each chromosome in a set 



has a definite length, thickness and shape, so that it can be recognized at 



* " Sat " stands for Sine Acido Thytrionucleinicus. 



