128 bulletin: museum of comparative zoology. 



The important feature at this stage, in its relation to later stages, is 

 the condition on the polar side of the chromatic mass. Each chromatic 

 mass is concave on the side toward the centrosome and convex on the 

 opposite side. The polar and equatorial surfaces of the chromatin lie 

 in the surfaces of two spheres whose common centre is marked by the 

 centrosome. These conditions point to the centrosome as the centre of 

 the forces by which the chromosomes have been moved toward and 

 grouped around the poles. 



Between the centrosome and the polar surface of the chromatin is a 

 region otherwise bounded by a conical surface extending from the 

 centrosome, as apex, to the nearer outer edge of the chromatic mass. 

 This is the region originally occupied by the end of the spindle, but at 

 this stage it is impossible to detect any fibres extending from the cen- 

 trosome toward the chromatin. This polar region is stained sufficiently 

 to be sharply outlined against the outer clear space of the cell, and yet 

 it is so much lighter than the chromatin as to be clearly distinguished 

 from the chromatic mass. It appears perfectly homogeneous, exhibiting 

 neither fibres nor granules. 



This condition of chromatin and polar structures persists for some 

 time while certain equatorial changes occur. These changes include the 

 formation of a membrane between the daughter cells (Plate 5, Figure 

 27). The figure meanwhile loses its barrel shape, assuming the form 

 of two cones with apices together at the plane of the new membrane. 

 The axis of the entire figure may become bent at the meeting of the 

 two cones. The axis of the left daughter cell in Figure 27 is almost 

 parallel to the plane of the section. The chromatic mass is of char- 

 acteristic appearance, but the centrosome was not clearly to be made 

 out. The axis of the right daughter cell extends obliquely upward, so 

 that the centrosome is obscured, and the observer looks down into the 

 concavity of the chromatic mass. The interzonal filaments are not 

 sharply constricted at the equator. The fact that varying degrees of 

 equatorial constriction of these filaments may be found, supports the 

 view that the fibres are carried inward toward the axis of the figure by 

 an ingrowing membrane. However, in most cells at this stage may be 

 found traces of fibres still occupying the region formerly filled by the 

 barrel-shaped figure. Such fibres are to be seen in Figures 27-29. 



Two facts of importance are to be noted at the stage of Figure 27. 

 The original cell membrane is in process of degeneration, the clear space 

 about the mitotic figure being no longer sharply outlined. It is of 

 irregular form, as if being encroached upon by the cytoplasm of near-by 



