farr: cytokinesis of pollen-mother-cells 293 



VI. The central spindles 



After the heterotypic karyoklnesis the central spindle persists 

 connecting the two daughter nuclei (fig. 21). Fibers may be 

 found running apparently the entire distance from one nucleus 

 to the other; they are attenuated near their ends, giving the 

 appearance of their being thickened throughout about one half 

 or two thirds of their length in the central region. Careful 

 examination, however, leads one to conclude that the fibers do 

 not pull away from the nuclei. Scattered about in the cytoplasm 

 are a few red-stained bodies, three of which are shown in the 

 drawing. The hyaline area about each is probably an optical 

 illusion due to diffraction. Comparison of this stage with a similar 

 one in the dividing cells of the onion root-tip reveals the fact that 

 in the latter the spindle fibers are somewhat more thickened 

 throughout one half of their length in the central portion, and a 

 very distinct orange zone and cell-plate have already begun to 

 form in the equatorial region of the spindle. In the mother- 

 cells of Nicotiana, however, no orange zone appears across the 

 equator, and no blue-stained plate is developed. The fibers 

 (fig. 22) continue to become more arched, as the mother-cell 

 approaches more and more the spherical form, and the rather 

 rudimentary spindle takes on a more inflated aspect. The peri- 

 pheral cytoplasm perhaps appears to be now more fibrillar; fibers 

 run out from the nucleus in all directions to the plasma membrane 

 and frequently cross along the surface of the spindle. There is 

 no evidence that these crossing fibers become transformed into 

 peripheral spindle fibers. In the later stages the fibers of the 

 central spindle proper seem not infrequently to cross each other, 

 whereas earlier (fig. 21) such a condition was quite exceptional. 

 It is difficult to state how this is brought about. There are 

 apparently, however, many more fibers present in the cytoplasm 

 in the later stages than immediately after the heterotypic karyo- 

 kinesis. 



The longitudinal axes of the homoeotypic spindles, as was 

 observed from the study of living material, are frequently at 

 right angles to each other (fig. 23). By the time the metaphases 

 of the second division have been reached, the heterotypic spindle 

 has entirely disappeared. The cytoplasm presents a densely 

 granular structure with evidence neither of alveoli nor of fibers; 



