118 INTRODUCTION TO CYTOLOGY 



directly through the attachment region, giving two chromosomes with 

 terminal attachment regions, is not certainly known, but the possibility 

 is strongly suggested by certain aberrations in Zea (p. 317). 



Nucleolus-forming Region. — In addition to the spindle-attachment 

 region the chromosome may exhibit one or more other differentiations 

 affecting its general morphology. Among these perhaps the most 

 interesting is the nucleolus-forming region, which seems as a rule to be 

 conspicuously developed in but one chromosome of a set. Although 

 it is reported to occupy the end of the chromosome in certain cases, it 

 is more commonly situated in a non-terminal position. Moreover, 

 its location is normally constant in a given race; like the spindle-attach- 

 ment region, it is a definite "organ" of the chromosome. 



The structure of this region is best known in sporocytes of Zea Mays, 

 where it appears at certain stages as a broad and chromatic region of 

 chromosome VI immediately adjacent to an achromatic region setting 

 off a "satellite," or small portion of the chromosome arm (Figs. 65, 170). 

 It is the chromatic region that is active in the accumulation of the 

 nucleolar matter in the telophase. This is shown by the fact that in 

 strains lacking the satellite, the achromatic region, and a portion of this 

 chromatic region, the nucleolus is seen to arise from the remaining 

 portion of the chromatic region. Normally, when chromosome VI is 

 present in its entirety, the growth of the nucleolus causes the achromatic 

 region to become extended as a slender filament lying over the nucleolar 

 surface, so that the satellite may be situated at some distance from the 

 remainder of the chromosome arm. In the ensuing prophase this 

 filament may shorten more or less as the nucleolus decreases in size and 

 disappears, but it remains visible as a conspicuous achromatic region, 

 or "secondary constriction."^ 



In most of the papers on this subject so far published it has been 

 stated that the nucleolus is associated with the "secondary constriction" 

 and satellite, the active chromatic region not having been recognized. 

 Thus S. Nawaschin and his associates'^ observed that during certain 

 phases of mitosis, notably the late prophase, the satellites lie on the 

 surface of the nucleolus (Fig. 64). In early prophases the satellite may 

 appear like any other equal portion of the extended chromosome (Fig. 

 65), but during the condensation of the chromosome the satellite typically 

 becomes a small spherical body (Figs. 64, 67, 72). In Viciafaha, investi- 

 gated by Heitz (1931a6), the somatic chromosome complement (Fig. 61) 



* Data in thi.s paragraph from McClintoek (1931& and unpublished). 



7 S. Nawaschin (1912, 1915, 1916 et seq.), Sorokin (1924, 1929), Baranov (1926), 

 Senjaninova (1926). SatelHtes were first described by S. Nawaschin (1912) for 

 Gallonia candicans. Wenrich (1916) observed the attachment of the nucleolus to a 

 certain portion of a chromosome in Phrynotettix. Kuhn (1928a) reviews the subject of 

 satellites and lists the plants in which they have been reported. 



