164 INTRODUCTION TO CYTOLOGY 



arrangement of materials about the chromosome suggests further that 

 the latter force is not resident in them but in the chromosome itself. 

 A hypothesis involving a progressive alteration at one or both ends of the 

 chromosome is advanced. 



Evidence for an active participation by the spindle is afforded by the 

 occasional division of nucleoli in normal bipolar mitosis. ^^ Ordinarily 

 the nucleolus has disappeared by the time the metaphase figure is estab- 

 lished in root cells, but frequently a portion of it may remain. In such 

 cases it tends to occupy the equator of the figure with the chromosomes. 

 It may then elongate and divide by constriction, the halves passing 

 toward the spindle poles, sometimes in advance of the chromosomes. A 

 similar behavior is manifested regularly by the ''karyosome" in the 

 nuclei of certain lower plants and animals (see Chapter XIII). In the 

 ascus of Pustularia the nucleolus is usually left outside the spindle, but 

 when it lies in the equator it divides. In sea-urchin eggs in hypertonic 

 media Konopacki (1911) observed that a whole nucleus might be sepa- 

 rated into two portions if caught between two cytasters. 



The foregoing facts seem to show that anaphasic chromosome migra- 

 tion is a process in which more than one of the nuclear constituents play 

 active roles. As the metaphase approaches, the prophasic karyolymph-'' 

 becomes organized as a spindle which is the visible expression of a field 

 of forces tending to divide and carry poleward any sufficiently fluid body 

 lying in the equator. The chromosomes, which are less fluid at this 

 stage, are specially adapted to reaction with these forces in being already 

 doubled when they enter the equator and in having specialized attach- 

 ment regions. Moreover, evidence cited above suggests an active force 

 resident in the chromosomes themselves. To the extent that the spindle 

 substance is actually a constituent of the chromosome {i.e., combined 

 with its matrix) during the metabolic stage, the mitotic division of the 

 nucleus may be regarded as a simultaneous and cooperative division of 

 its several individual chromosomes. In amphiastral figures the process 

 is closely correlated with the multiplication of centrioles and a series 

 of structural alterations in the surrounding cytoplasm, such figures 

 comprising both nuclear and cytoplasmic elements. ^^ The nature and 

 relative importance of the forces involved are still obscure. It seems prob- 

 able that mitosis involves the action of many factors, including alterations 

 in viscosity, 22 surface tension, and permeability, as well as streaming, 

 tension, and electrical attractions and repulsions. In no one of these 

 factors alone will the entire solution of the problem of mitosis be found. 



" Nemec (1901) on Alnus, Yamaha and Sinoto (1925) on Glycine and other genera, 

 Zirkle (19286) on Zea, Frew and Bowen (1929) on Cucurbita and Pustularia, Ghimpu 

 (1930) on Acacia. 



2" The "parachromosomic substance" of Koerperich (1930); the "paragenoplast" 

 of Bleier (1930c). See discussion by Wilson (1932). 



21 See Wilson (1925, pp. 174-178). 



22 Chambers (1917), Heilbrunn (1920a, 1921). 



