92 



CYTOKINESIS. 



filled with a clear, non-staining substance. The regular spreading of this old sphere 

 substance over the pole of the spindle is an actual demonstration of Biitschli's ('92, 

 1900) view that the poles of the spindle represent diffusion centers, from which sub- 

 stances spread over the surface of the cell. There is no conclusive evidence, how- 

 ever, that this diffusion consists -of centrifugal movements within the astral rays 

 themselves, since the spreading of the old sphere substance might be brought about 

 by centrifugal movements of the substance between the rays or by peripheral move- 

 ment of the entire spindle; there is actually such a movement of the spindle, as has 

 been described already (p. 83). 



Fig. XXVIII. 

 Pigs. XXVIII, XXIX.— Late stages in the third and fourth cleav 

 :e substance in the form of a ring at the upper pole of the spindle. 



Fig. XXIX. 

 s of Crepidula showi 



Z the spreading of the old 



In the late anaphase the sphere substance is again aggregated over the pole of 

 the spindle ; this is accompanied by a polar elongation of cytoplasm and the conse- 

 quent separation of the pole of the spindle from the cell wall, together with the 

 reduction of the angle of the "antipodal cone" mentioned above; at the same 

 time the old sphere material is drawn in toward the centrosome, frequently in the 

 form of a funnel, until its remnants lie close over the centrosome and new sphere, 

 text figs. XXII, XXIII. It seems probable that the aggregation of the old sphere 

 substance, the withdrawal of the astral rays and the' coincident growth of the 

 daughter spheres are all dependent upon centripetal movements along the rays 

 during the anaphase. 



There is good evidence in favor of the view that the astral rays are absorbed 

 directly into the sphere in the later stages of mitosis. Thus the sphere increases in 

 size and becomes rounded in outline as the astral rays diminish, and though the 

 radiating arrangement of the alveoli may persist right through the resting stage 

 (figs. 61, 69), the substance of the rays has largely disappeared when the spheres 

 have reached their maximum size. Furthermore, after the separation of the 

 daughter cells the spheres always become proportional in size to the volume of the 

 cells in which they lie. Now, the first recognizable difference in the structure of 

 unequal daughter cells is found in the size and extent of the astral rays; the 



