GERM-CELL HISTORY IN THE BROOK LAMPREY 53 



A little later (fig. 43) the chromatin masses become still more 

 conspicuous, but they continue to be held together by the 

 achromatic threads. The nucleoli are still large, but generally 

 one is considerably smaller than the other. They are closer 

 together than in the preceding stage. 



The chromatin granules or masses soon become separated from 

 one another by the breaking of the achromatic threads which 

 up to this time have been holding them together (fig. 44) . Each 

 mass is now clearly a short globular chromosome. It was im- 

 possible, however, to count the chromosomes with any degree 

 of accuracy, either at this or any other stage. At this time only 

 one nucleolus is visible. The nuclear membrane is still intact. 

 There is some indication that the chromosomes are arranged in 

 pairs. 



In the next stage (fig. 45) the nuclear membrane disappears 

 and the chromosomes come together in a mass at about the 

 equator of the cell. The two centrosomes occur at opposite 

 poles, and achromatic spindle fibers extend from the centro- 

 somes to the chromatin mass. The nucleolus is no longer visible 

 and its fate is not known. A polar view of this stage is shown 

 in figure 46. The chromosomes are so closely massed that it is 

 difficult to distinguish each individual chromosome. 



Metaphase. Although numerous dividing cells were exam- 

 ined, none was found in which the splitting of the chromosomes 

 could be observed. It seems likely that this process takes place 

 so rapidly that the chances of finding a cell in this stage are 

 slight. Furthermore, the chromosomes are so closely massed 

 together on the equatorial plate that their division would be 

 difficult to observe. 



Anaphase. Figures 47 and 48 represent cells in early anaphase. 

 The daughter chromosomes have already begun their migration 

 to the opposite poles. Occasionally bodies having the appear- 

 ance of chromosomes are found outside of the spindle in the 

 cytoplasm of the cell, or they may be scattered along the spindle 

 threads. 



Telophase. In this stage (figs. 49 and 50) the daughter chro- 

 mosomes have separated from each other and form irregular 



