726 CALKINS. [Vol. XV. 



gates and bends in the plane of the primary axis, the central- 

 spindle sinks into the depression which is thus formed, until it 

 finally occupies the position x in the Fig. ©. The spindle thus 

 lies in the secondary axis of the nucleus which encircles it, the 

 spheres alone remaining outside (Figs. 13-15). The nuclear 

 plate is wrapped around the spindle like a ring, the chromosomes 

 lying midway between the two poles. The central-spindle 

 fibers are at first not straight, but run from pole to pole in 

 curved lines (Fig. 1 1). This curvature is caused by the spheres 

 moving around the nucleus through an arc of 90°, the fibers 

 becoming straight only when the central-spindle finally lies in 

 its definitive position (Fig. 13). In some cases the bend of the 

 central-spindle is much exaggerated until an acute angle is 

 formed. During this time the growing chromosomes are 

 entirely separated from the central-spindle, but when the 

 nuclear plate is complete the intervening membrane disappears, 

 and then, as in the metazoan spindle, the chromosomes lie 

 directly upon the central-spindle. After the preliminary 

 arrangement of the elements of the mitotic figure, the central- 

 spindle undergoes a considerable elongation (Figs. 17-19, 30), 

 while at the same time the nucleus is drawn out in the direction 

 of its secondary axis in the form of a hollow cylinder with a 

 ring of chromosomes at either end (Figs. 17-19). The dumb- 

 bell-shaped amphiaster lies in the hollow with the spheres 

 projecting at either end (Fig. 13). After division of the vege- 

 tative nucleus the sphere returns to its resting condition (Fig. 

 20). It becomes less dense and less homogeneous, and the 

 central part more or less hyaline, while the peripheral portion 

 still retains its granular aspect. The transformation may begin 

 even before the nuclei are completely separated. Such a con- 

 dition is indicated in Fig. 20, where the daughter-spheres, having 

 lost their densely granular spherical condition, have become 

 more diffuse, and have acquired the characteristic appearance 

 of the resting spheres. On the other hand, after spore-forming 

 divisions, the daughter-spheres do not return to the resting 

 condition. They retain their spherical shape and dense granu- 

 lation (Figs. 17-19), and soon divide again for the ensuing 

 mitosis. It frequently happens that this secondary division of 



