THE MORPHOLOGY OF THE CHROMOSOMES 



119 



includes two long members with conspicuous achromatic regions setting 

 off segments larger than those usually called satellites. Owing to the 

 position assumed by the chromosomes in late anaphase these two regions 

 lie at a certain level in each telophase nucleus, and a nucleolus develops 

 near each of them. If the two chromosomes concerned lie far apart 

 in the nucleus the two nucleoli remain separate, but if they lie near 

 each other the nucleoli may come into contact and fuse. Hence some 

 nuclei in the root show two nucleoli wdiile others show one, and as a 

 rule sister nuclei recently formed by division are mirror images of each 

 other. This feature is represented in Fig. 54. In Zea roots the nucleolus 

 is seen to be connected with the developing chromosomes at two points 



Fig. 64. 



Fig. 65. 



Fig. 64. — Satellited chromosomes attached to nucleolus in Galtonia. [After S. 

 Nawaschin, 1927 (1913).] 



Fig. 65. — Chromosome VI (synapsed pair) attached to nucleolus in microsporocyte of 

 Zea Mays. The satellite projects upward from the chromatic swelling against the nucleolus. 

 A short distance below the swelling is the spindle-attachment region, which here appears 

 like a gap. Note chromatic knob toward left end. Compare Fig. 170. {After McClintock, 

 1931b, 1933.) 



(Zirkle, 19286); these are presumably the nucleolus-forming regions of 

 the two chromosomes VI. 



Because of the mode of their formation, it seems that nucleoli are as 

 a rule connected with certain parts of certain chromosomes in the nuclear 

 reticulum. At present too little is known to warrant anything more 

 than suggestions regarding the significance of this phenomenon. It 

 has long been thought by many observers that some nucleolar constituent 

 passes into the chromosomes in the prophase and out of them in the 

 telophase. This view is supported by the appearance of nucleoli as 

 the chromosome matrix diminishes in amount and loses its chromaticity 

 in the telophase, their disappearance as the chromaticity returns and 

 the matrix becomes abundant in the prophase (Martens, 1922), and 

 the results obtained with special fixing reagents (Zirkle, 19286). Fre- 

 quently it seems that only one of two principal nucleolar constituents 

 functions in such a manner (Selim, 1930). If there is such a transfer 



