12 bulletin: museum of comparative zoology. 



often are so closely crowded in the narrow limits of the cell that the 

 individual elements cannot be distinguished. 



The archoplasmic structures at this stage are well developed for such 

 small cells. The centrosomes are minute, but very distinct, dots, about 

 which the cytoplasm is slightly more dense, forming rudimentary cen- 

 trospheres. The spindle fibres are numerous — apparently more so than 

 the chromosomes — and take a dark stain. The archoplasm is arranged 

 about the centrosomes in a radiate manner, forming faint astral rays. 



In later stages the cell begins to elongate, the lengthening of the cell 

 being accompanied by a corresponding change in the form of the spindle 

 (Fig. 5). Accompanying this elongation of the spindle, the chromo- 

 somes are divided and immediately drawn toward the poles (Fig. 5). 

 "When this takes place the interzonal filaments, as usual, are seen extend- 

 ing between the separating groups of chromosomes. This is not strange 

 in view of their persistence and marked behavior in the succeeding stages. 

 In Figure 6 is shown an oblique polar view of a cell in the late anaphase. 

 The chromosomes are grouped in a closely packed mass at the pole, and 

 the spindle fibres present the appearance of radiations of the cytoplasm 

 centering in this chromatin mass. 



When the divided chromosomes have become aggregated in dense 

 masses at opposite poles (Fig. 5), the cell lengthens still more and 

 immediately begins to constrict. The constriction is never entirely com- 

 pleted at this time, for, near the centre of the wall between the two cells, 

 there always remains a small round opening (Figs. 7-12). By this con- 

 striction of the cell the interzonal filaments are forced together into a 

 sheaf-like bundle extending through the opening in the constricting wall 

 and with one extremity in each cell. Immediately after the separation 

 of the chromatin has been accomplished, the centrosome divides into two 

 small bodies, and at the stage represented in Figure 7, these have become 

 surrounded by a mass of archoplasm. This is of a very finely granular 

 consistency, and undoubtedly is formed by the breaking down of the 

 astral rays and the polar ends of the spindle fibres. 



When the chromosomes migrate to the poles of the cell, they are 

 grouped into such a dense mass that the outlines of the individual ele- 

 ments cannot be distinguished. Soon, however, this mass begins to 

 show signs of important change. Processes arise from the surface, and 

 soon (Figs. 7, 8) the outlines of many of the individual elements can 

 be distinguished. It is apparent that they are lengthening and be- 

 coming diffuse and granular. At places, however, they are still so 

 closely grouped that individual outlines cannot be traced. One of these 



