THE MORPHOLOGY OF THE CHROMOSOMES 121 



normal cycle, this in turn suggesting why practically every chromosome 

 set, well studied, shows at least one achromatic region, or "secondary 

 constriction," which so often lies near a nucleolus-forming region. 

 It is known, however, that nucleoli may develop at other points in 

 the chromosome set under certain circumstances, as when the nucleolus- 

 forming region has been removed in the deletion of a portion of the 

 chromosome. Thus when a strain of Zea Mays in which one member 

 of chromosome pair VI had lost this region produced microspore quartets, 

 it was found that two spores of each quartet had the region and a single 

 large nucleolus, while the other two had no such region and a considerable 

 number of small nucleolus-like bodies (Creighton) (Fig. 65a). These 

 small masses of substance, which would normally contribute to the forma- 

 tion of a single nucleolus, may appear at one or more points on each 

 chromosome. Heitz (19316) reports that when a small extra nucleus 

 is formed by a lagging chromosome in Vicia, it may contain a small 

 nucleolus in addition to the two normal ones in each main nucleus. 

 From such facts it may be concluded that all of the chromosomes give 

 off a material during the telophase, and that under normal circumstances 

 this material is incorporated in typical nucleoli formed in close associa- 

 tion with specialized regions of certain chromosomes. 



Other Specialized Regions. — Besides the spindle-attachment region 

 and the nucleolus-forming region, other differentiations can frequently 

 be observed in the chromosomes at metaphase and anaphase. These 

 may sometimes appear as "constrictions" (Fig. 61), but very little is 

 known about the functions they may perform or concerning the number 

 of kinds of them which may be present in a chromosome set. It seems 

 improbable that constrictions are necessarily places where fragments of 

 chromosomes have formerly joined,^ although translocated pieces in 

 Drosophila are sometimes set off by narrow regions. 



The Chromosome Complement. — In any given kind of plant or 

 animal each nucleus contains an outfit, or complement, of chromosomes 

 composed of a certain number ^° of members showing characteristic 

 differences in form and function. As a general rule, the nucleus of an 

 egg before fertilization contains a complement made up of one each of 

 several kinds of chromosomes. Such a complement is called a set, or 

 genom. Because each kind of element is represented but once, a nucleus 

 (or organism) with such a chromosome outfit is said to be mono-ploid 



^ M. Nawaschin (1931c), Lewitsky and Araratian (1931). 



1" The known chromosome numbers in all plant groups are listed by Tischler 

 (1927, 1931). Gaiser (1926, 1930) gives lists for angiosperms. See also Aase and 

 Powers (1926) and Sisa (1929) for crop plants, Stolze (1926) for grains, Avdulow 

 (1931) for Gramineae, Kawakami (1930) for Leguminosse, and Kihara, Yamamoto, 

 and Hosono (1931) for plants cultivated in Japan. For animals, see Bresslau and 

 Harnisch (1927) and Harvey (1916, 1920); also Oguma and Makino (1932) on verte- 

 brates. Wilson (1925) gives a fairly extensive list for plants and animals. 



