THE CHROMOSOMES 



97 



differences in band pattern, so that it becomes possible to assign contribut- 

 ing causes of certain characters to particular chromosomal regions. Thus 

 the salivary-gland chromosome map becomes to the specialist a sort of 

 biological spectrum indicating the organism's genetical constitution, 

 much as an absorption or a bright-line spectrum reveals the chemical 

 composition of an inorganic body. 



Chromosome Complements. — Any group of chromosomes composing 

 a nucleus, whatever their number or kind, is a chromosome complement. 

 The simplest typical complement is one made up of several members 

 differing variously in form and function but acting together as a complete 

 and harmonious system; such a complement is a genome, or set. Since 



a< 



— .—— o- 



■oooo— •■— -co 



^^oO^^CX> 



Fig. 70. — Drawings and diagrams illustrating (a) the theory that the salivary-gland 

 chromosome consists of many reduplicated chromonemata with their chromomeres and 

 (6) the theory that it has an alveolar structure with chromatic matter variously distributed 

 within it. (a, after T. S. Painter and A. Griffen; b, after C. W. Metz.) 



only one chromosome of each kind is present, the nucleus (or tissue, or 

 organism) with such a complement is said to be monoploid. 



A nucleus may contain one to many genomes. In typical cases, 

 sexually reproducing organisms exhibit an alternation of two chromosome 

 numbers in the course of the life cycle. Two gametes, each monoploid, 

 unite to form a zygote W'hich has Qvavy kind of chromosome in duplicate 

 and is therefore diploid. In higher animals and plants this diploid con- 

 dition is maintained by equational mitoses throughout the development 

 of the body. When the animal produces gametes, the diploid number 

 is reduced by the process of meiosis to the monoploid number, each 

 gamete having a single complete genome which may include members 

 from both of the original genomes. In the plant this reduction in number 

 occurs when spores are produced. The monoploid number is then 

 maintained through the development of the gametophyte and the 



