THE POLYTENE NUCLEUS 



four, eight, sixteen, and higher doublings of threads. Each of these 

 threads, when stained, has a characteristic beaded structure, and the 

 identical beads of the multiplied sister threads form plates. Each 

 polytene, or multiple chromosome, then looks like a banded ribbon 



CENTROMERES and HETEROCHROMATIN 



IV 



Fig. 5. — The paired and banded polytene chromosomes of a pressed saUvary gland 

 nucleus in Drosophila mclanogastcr. AU the centromeres and heterochromatin are 

 agglutinated in a single body at the top. The two arms of each of the long autosomes 

 are marked. X ea. 600 (after Painter 1934). 



as seen from the side. Now these bands, of which there are hundreds 

 in each chromosome, are different from one another, and constant 

 in size, structure and number, just as are the chromosomes in a 

 metaphase plate. They are the chromomeres (Fig. 5). 



The linear order of the chromomeres is recognizably constant 

 from nucleus to nucleus and displays the individual permanence of 

 the chromosomes. It shows us how they must maintain their struc- 

 ture as a line of distinct particles within the resting nucleus. It also 



27 



