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INTRODUCTION TO CYTOLOGY 



tinue after cytoplasmic streaming has been stopped, and also that the 

 streaming may continue when mitosis is arrested. The appearances in 

 living and fixed cells also suggest a longitudinal tension of some kind in the 

 figure, or even a bundle of tubes through which the chromosomes move 

 (Reuter, Schrader). 



As a result of his investigations on the living spermatocytes of Chort- 

 hippus (Stenobothrus) and stamen hairs and leaf-cells of Tradescantia, 

 Belaf advanced the following hypothesis, which is a modification of an 

 earlier one advocated by Drliner (1895) and others, to account for chromo- 

 some movement (Fig. 91). The spindle, 

 which develops progressively from the two 

 poles to the equator, is composed of relatively 

 firm fibers (which are aggregates of micellae) 

 and a less viscous substance between them. 



Fig. 90. Fig. 91. 



Fig. 90. — Modifications of living anaphase spindles in Chorthippus induced by immers- 

 ing the spermatocytes in hypertonic Ringer's solution, a, b, elongation and contortion, 

 c, splitting. The white areas represent the chromosomes. (After Belaf, 1929a.) 



Fig. 91. — Diagram illustrating Belaf's hypothesis of interaction of chromosomes and 

 spindle. See text for explanation. {After Belaf, 1929a.) 



The double chromosomes become fixed in or on the spindle at its 

 equator by their attachment regions. From this region in each half- 

 chromosome there is exuded a fluid droplet which spreads poleward along 

 the spindle; this is the "tractile fiber" of other authors and it does not 

 grow toward the chromosome from outside the nucleus as often supposed. 

 The separation of the daughter chromosomes (or of synaptic mates) is 

 initiated by the chromosomes themselves, the tractile fiber mechanism 

 assisting in some unknown manner, perhaps through forces of adhesion 

 acting between fiber and spindle; but after this the continued separation 

 is due mainly to the elongation of the mass of spindle substance ("Stemm- 

 korper") lying between the diverging groups of chromosomes. In 

 spermatocytes immersed in hypertonic media this elongation may be 

 caused to continue until the mitotic figure is far beyond its natural length 

 and lies folded together within the confining cytoplasm (Fig. 90, a, b). 



