50 bulletin: museum of comparative zoology. 



prediction, but there can be no doubt whatever that such a phenomenon 

 occurs (Figs. 159, 160). By these changes the cones of cytoplasm at 

 the poles of the division figure are very much decreased in size, but are 

 still apparent (Figs. 66, 67) ; indeed, they still continue to be visible up 

 to the time of the early telophase. 



The next changes which take place in the cell concern the division of 

 the chromosomes. This is quickly accomplished, the groups of daughter 

 elements beginning at once to move toward the opposite poles of the cell. 

 This movement is apparently brought about, not entirely by the con- 

 traction of the fibres connecting the poles of the spindle with the chro- 

 mosomes, but likewise by the contraction of those uniting the centrosome 

 and the apical point, for in all stages succeeding the anaphase (Fig. 67), 

 the mantle fibres, as long as they persist, centre in the usual structure 

 (Figs. 68, 69). Each group of chromosomes, as it approaches its desti- 

 nation, collects into a dense mass in the usual manner, as shown in Figure 

 68. These masses of chromatin do not reach the centrosomes, but remain 

 at the region which the apical point formerly occupied. 



When the chromosomes have reached this point, the centrosomes again 

 move apart and take up their final position directly in contact with the 

 inner face of the cell membrane. By this last movement the cytoplasmic 

 cones, visible up to this time, are of course entirely obliterated (Fig. 69). 

 We have seen that during the metaphase and early anaphase the centro- 

 some and centrosphere are elongated in the direction of the axis of the 

 spindle (Figs. 54, 65-67). At the completion of the anaphase this is 

 no longer true. When the migration of the daughter masses of chro- 

 matin is completed, the centrosphere has lost its conical shape and is 

 spherical or oval. It is worthy of note that after the retraction of the 

 fibres connecting centrosome and apical point the centrosphere increases 

 very appreciably in size. This affords still further evidence of the for- 

 mation of the centrosphere from archoplasmic material. When the 

 anaphase is entirely completed, and the centrosome has assumed its final 

 position in contact with the cell membrane, it is seen that the centro- 

 some also has changed form (Figs. 55, 69). It is now a dumbbell- 

 shaped body with its long axis at right angles to its former direction, 

 i. e. it is now parallel to the cell membrane. 



The phenomena connected with the division of the cell body of the 

 small spermatocyte are similar to those already noted for the large type. 

 However, the behavior of the centrosomes and archoplasmic rays indicate 

 even more strongly than in the other type the fundamental influence of 

 these structures upon cytoplasmic cleavage. I have noted above that 



