22 LABORATORY DIRECTIONS IN 



part with lines radiating in all directions into the cytoplasm. Each 

 part is an aster. The two asters may be connected by other lines, the 

 double structure being the amphiaster. If only one aster is present, how 

 do you account for the absence of the other? Draw a or b. 



2c. Find cells in, which the nuclear membrane has disappeared, and 

 in which the chromosomes, now quite thick and distinct, have no definite 

 arrangement. Count the chromosomes. Look in the cytoplasm for 

 an amphiaster. Each aster should contain a central granule, the centro- 

 some. If only one aster is seen, where is the other? Draw. 



2d. In a later stage the chromosomes are arranged in a flat group. 

 Seen on edge, they form a nearly straight line; viewed from the flat side 

 of the group, the chromosomes are readily distinguishable. How many 

 are there? This group of chromosomes is the equatorial plate. In a 

 cell in which the equatorial plate is seen on edge, note the two attraction- 

 spheres and centrosomes (that is, observe the amphiaster). The lines 

 connecting the asters with the chromosomes and with each other are 

 called spindle fibers. 



Draw a cell with the spindle in side view; that is, with the equatorial 

 plate seen on edge. Draw another cell to show the equatorial plate as 

 viewed from one of the centrosomes. 



3. Metaphase. While in the equatorial plate, or earlier, the chromo- 

 somes split longitudinally. If you do not find this stage readily, ask 

 for a demonstration either in Ascaris, or in the skin of a salamander, or 

 in some other cell. 



4. Anaphases. Find cells in which the halves of the divided chromo- 

 somes have begun to separate into two groups of chromosomes. If pos- 

 sible, count the chromosomes in each group. Note the form of the 

 spindle. Are there fibers between the groups of chromosomes? Draw 

 either an early or a late stage; that is, one in which the groups of 

 chromosomes are still near together or are widely separated. 



5. Telophases. Search for a later stage than 4, showing the two 

 groups of daughter chromosomes separated by a cell membrane which 

 has divided the original cell into two cells. Is the nuclear membrane 

 present around the groups of chromosomes? Are the centrosomes 

 visible? Do any signs of the spindle remain? Draw. 



C. SUMMARY 



Give a brief but clear account of the whole process of mitosis. Do not 

 treat it as a series of stages, like the ones you have studied, but as a con- 

 tinuous process. That is, fill in the gaps between the stages studied, 

 using any reliable source of information. 



