86 GARDINER. [Vol. XV^ 



later this vesicle becomes dumb-bell-shaped and breaks into 

 two bodies which (Fig. lo) for a time are connected by fila- 

 ments. Apparently one of these bodies remains stationary 

 while the other migrates through an arc of i8o° to the other 

 side of the nucleus, for, as Fig. 1 1 shows, one of the poles of 

 the amphiaster is formed near the broken-down nucleolus, at 

 which point the centrosphere first appeared. 



In regard to the rays which extend out from these vesicles, 

 it is difficult to say whether they are composed of the same 

 substance as the walls of the vesicle, and therefore a part of it, 

 or whether they are formed directly from the cytoplasm in 

 which the centrosphere is imbedded. As, however, they grow 

 in length as the vesicle increases in size, both the vesicle and 

 rays must draw sustenance from the cytoplasm. These rays 

 seem to grow at either outer end by a direct change of the 

 cytoplasm, as Wilson (14) has described as occurring in the 

 fibers about the sperm asters of Toxopeneutes . 



As soon as the centrospheres have attained their positions 

 at opposite sides of the nucleus the rays become speckled 

 with numerous fine microsomes, and at the same time lengthen 

 out so as to come in actual contact with the surface of the 

 nucleus, which at those points becomes very irregular in con- 

 tour (Fig. 11). It appears as if the ends of the fibers were 

 exerting mechanical pressure on the surface of the nucleus 

 and bending it in, but at the same time it is very evident 

 that the nuclear membrane is being dissolved at these points. 

 In a stage somewhat later than that shown in Fig. 1 1 

 the membrane is distinctly thinner, while in still later stages 

 (Figs. 12 and 13) it has entirely disappeared, leaving no 

 fragments in these regions, though it long continues round 

 the rest of the nucleus. With the first disappearance of the 

 membrane in this region, the substance of which the achromatic 

 spindle fibers are to be formed appears. Apparently it is the 

 linin of the nucleus which on the dissolution of the nuclear 

 wall flows out toward the centrosphere (Figs. 12 and 13). 

 At this stage it is homogeneous in appearance and does not 

 show the fibrous structure which characterizes it later. It 

 seems to flow directly out of the nucleus, and while so doing 



