126 bulletin: museum of compakative zoology. 



the earliest condition of the chromosomes was that seen in Figure 21, 

 where the chromatin is in the form of large, nearly spherical granules. 

 The number of chromosomes could not be accurately determined. 



The presence of a centrosome was not observed until the spindle had 

 been well formed. The two granules lying at the edge of the chro- 

 matic mass in Figure 21 (cell at the right) may be the centrosomes 

 which have not yet taken their position at opposite poles. Cells were 

 found where the two poles of the spindle had been established, and 

 the chromosomes were not yet aggregated into an equatorial plate. 

 Sometimes an extremely minute centrosome could be distinguished at 

 the poles of the spindle at this stage. When the cell is in the meta- 

 phase the centrosomes are generally conspicuous objects, although there 

 is much variation as to their size. The half spindle in Figure 21 (cell 

 at the left) shows a centrosome. In Figure 22 the centrosomes are 

 unusually large for cells at that stage. The average size of the centro- 

 somes is perhaps fairly indicated by Figure 24. 



In the metaphase the cell membrane is still present. The loss of a 

 perfectly spherical form may mean a decreased internal pressure. Par- 

 ticular attention is directed to the fact that there is very little stained 

 material outside the limits of the spindle. Figure 23 represents a very 

 typical metaphase. Except for a slight cloudiness near the poles, the 

 cell body is quite clear. The spindle is very sharply outlined, and at its 

 poles are minute deeply stained centrosomes. No polar radiations are 

 visible. In Figure 22, except for a few indefinite irregular masses of 

 unstained material, the body of the cell is clear. In this case, however, 

 a few very indefinite polar rays could be seen. In Figure 24 there is an 

 unusually large amount of material, apparently finely granular, in the 

 region of the two poles, and into this material extend some well defined 

 polar radiations. The half spindle of Figure 21 shows an extreme case 

 of the presence of solid substance outside the spindle. 



In the metaphase, then, the body of the cell is generally clear. If 

 solid material be present outside the limits of tlie spindle, it tends to 

 be accumulated about the poles and may be associated witli polar 

 radiations. 



The division of the chromosomes must take place at a very early 

 stage of the equatorial plate, for in most of the cells in the metaphase 

 the chromosomes are in a double layer. 



The region occupied by the spindle always presents a compact and 

 homogeneous appearance, although not deeply stained. A limited 

 number of conspicuous fibres may be seen lying upon the surface of the 



