NUCLEUS 



225 



nucleoli condense during the telophase. Moreover Fig. 125 a shows 

 some heterochromatic parts (end of left chromosome arm and satellite). 

 An obvious hypothesis relating to the submicroscopic structure of 

 the chromosomes, deriving support from Heitz (1935) and Geitler 

 (1934, 1938) in their elaborately documented summarizing studies on 



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Fig. 123. Microscopic chromosome structure (from Heitz, 1935). a) Idealized chromo- 

 some with helicoid chromonema threads; heterochromatic region hatched; in the upper 

 part : a primary (kinetic) constriction ; in the lower part at the right : secondary constriction 

 with satellite (corrected to satisfy Geitler's criticism 1938, p. 98). b) Chromatid pair of 

 Trillium erectum. c) Spiral structure of the chromosomes of Tradescaiitia virginica. 

 d) Spiral structure of the chromosomes of Trillium erectum. 



the structure of chromosomes can be built up from the chromonema 

 theory. Each chromosome contains two, according to other investi- 

 gators (Nebel and Ruttle, 1937; Nebel, 1939, 1941) even four 

 spirally wound threads, called chromonemala (Fig. 123 a). In the first 

 case they are identical with the chromatid threads (Fig. 123 b), well 

 known from the prophase of meiosis. It is only in that prophase that 

 the chromonema spiral is completely uncoiled and therefore survey- 

 able in its entire length, which is many times that of the chromosome. 

 It consists of a non-staining thread which at regular intervals is covered 

 with knots showing the nucleal reaction and designated as chromo- 

 meres. In the mitosis chromosomes these particulars can scarcely be 

 observed, as the chromonemata are coiled (Fig. 123 c) and embedded 



