GROWTH AND CELL DIVISION 77 



region and conical poles (e). The spindle fibers are supposed 

 finally to pass through the nuclear cavity from pole to pole of the 

 complete spindle. When the spindle began to form, the chromo- 

 somes occupied the periphery of the nuclear cavity next to the 

 nuclear membrane. As the half spindles elongate across the 

 nuclear cavity with the disappearance of the nuclear membrane 

 the chromosomes appear to be pushed into the equatorial region of 

 the nucleus between the two half spindles, forming what is some- 

 times called the equatorial plate. The chromosomes of this equa- 

 torial plate ultimately move toward the outside of the completed 

 spindle and become arranged in a definite radiate manner at its 

 periphery, attached to the outer spindle fibers. On account of 

 their radiate appearance at this stage it is often called the mother 

 star stage (/). The particular fibers to which the chromosomes 

 are attached are called the traction fibers, since, as we shall learn, 

 they appear to contract and separate the two halves of each 

 chromosome in the next stage of mitosis. The remaining fibers 

 of the spindle are termed the central spindle fibers, or supporting 

 fibers. With the completion of the spindle and the arrangement 

 of the chromosomes in the mother star stage the events of the pro- 

 phase are completed and those of the metaphase_are ushered in. 



The metaphase stage (/) is concerned with the equal longitu- 

 dinal division of the chromosomes into half chromosomes and with 

 the separation of these half chromosomes preparatory to the mi- 

 gration of the chromosomes to the poles of the spindle to form the 

 daughter nuclei. In this process each chromosome splits through- 

 out its entire length into two equal halves, each of which appears 

 to be attached to a traction spindle fiber. The half chromosomes 

 then begin to separate, as though pulled apart by the shortening 

 traction fibers. The final separation of the half chromosomes 

 marks the end of the metaphase stage. 



The anaphase is the stage (g) in which the half chromosomes 

 continue their migration toward the poles of the spindle, where 

 they finally arrange themselves in a more or less radiate manner 

 resembling somewhat the mother star stage of metaphase. This 

 radiate arrangement of the daughter half chromosomes is there- 

 fore called the daughter star stage and marks the end of anaphase 



