CELL-DIVISION 



events of the first cleavage-cycle only, my account will be 

 limited similarly. This record of the happenings in the 

 living egg is supplemented by that obtained through study 

 of the fixed Q%%. 



Let us begin the account of the first cleavage-mitosis in 

 the Q'g% of Arbacia with the stage when the two germ-nuclei 

 have disappeared by complete fusion to form a single resting 

 nucleus, a clear vesicle almost spherical in shape lying in the 

 polar axis just above the equator of the egg. At this 

 moment two centrospheres with asters are present at oppo- 

 site poles of the nucleus; the egg is spherical, oil-drops and 

 yolk-spheres, lying outside of the spindle-area, are evenly 

 dispersed, the pigment granules are trapped at the egg- 

 surface and the hyaline plasma-layer is clearly defined. 



In the process of cleavage we shall follow especially these 

 changes: the growth of the centrospheres and asters, the 

 varying shape of the eg^, the shifting of the cytoplasmic 

 inclusions and the activity of the hyaline plasma-layer as 

 the nucleus goes through its mitotic cycle. These four 

 changes are easily visible and readily followed in the living 

 egg. In accordance with the principle hitherto followed, I 

 shall endeavor to derive a conclusion as to the meaning of 

 the process by relating the events that can be observed. 



The nucleus seen in the living egg does not retain its 

 spherical form; increasing in size it becomes ellipsoid and 

 ruptures at the poles of its long axis where the centrospheres 

 and asters are located. A row of hyaline droplets, the 

 chromosomes, extends across the equator of the transparent 

 ellipsoid nuclear region; soon these form two parallel rows 

 which slowly separate as the ellipsoid nuclear region 

 by a constriction at the equator becomes dumb-bell 

 in shape. As each group of droplets approaches the inner 

 border of the centrosphere (the centrospheres are now hemi- 

 spheres presenting their plane-surfaces toward the dumb- 

 bell shaped clear region) they become spherical. Suddenly 



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