THE PROCESS OF CELL DIVISION 63 



of these is a rounded vesicle, inclosing the usual chromatin granules and nuclear 

 sap. Draw such a stage in detail. The egg shell may be omitted. 



2. Spireme stage or early prophase. As cell division begins the chromatin 

 granules thicken and mass together, finally uniting into a long, coiled thread, 

 called the spireme, which fills the nucleus. Consider that in sections only pieces 

 of such a coiled thread could appear. Look, therefore, for nuclei containing 

 deeply stained elongated pieces of chromatin. At this time also there appear 

 in the cytoplasm two dense collections of granules, called the asters. Each 

 aster has in its center a black granule, the centrosome, and sends out radiations, 

 the astral rays, into the surrounding cytoplasm. A complete picture of the 

 spireme stage shows the egg and sperm nuclei, containing broken threads of 

 chromatin, in contact with each other, and an aster placed at each end of the 

 plane of contact. You may not be able to find a cell cut in the right plane to 

 show all of these parts simultaneously. Draw in detail the best example you 

 can find. 



3. Late prophase. The spireme thread now breaks into a number of distinct 

 threadlike bodies, each of which is called a chromosome. In Ascaris there are 

 four of these. The nuclear membrane has disappeared and the chromosomes 

 lie free in the cytoplasm. They stain very deeply, and are usually U-shaped. 

 Meantime the asters have drawn farther apart and delicate fibrils extend between 

 them. These fibers are called the spindle, and the whole structure, asters and 

 spindle, is known as the mitotic figure. Draw a cell showing the four chromosomes 

 free in the cytoplasm. 



4. Metaphase. The chromosomes now arrange themselves in the center 

 of spindle. The mitotic figure is now fully developed and symmetrically placed 

 in the cell. Find a cell which is cut parallel to the spindle and draw, showing 

 spindle, asters, and the band of chromosomes across the center of the spindle. 

 Each chromosome has a longitudinal split at this time, which is generally difficult 

 to see. 



5. Anaphase. Each chromosome next splits in two longitudinally and the 

 two halves separate. One-half of each chromosome moves toward one aster 

 and the other half to the other aster. In this migration the ends of the dhromo- 

 somes always point toward the middle of the spindle, so that the dividing cell 

 in this stage contains two groups of chromosomes, each group looking something 

 like the top of a palm tree, with the delicate parallel threads of the spindle 

 stretching between them. Draw. 



6. Telophase. The chromosomes approach the asters, where each group 

 condenses into a mass in which the individual chromosomes are no longer dis- 

 tinguishable. A constriction which gradually pinches the cell into two equal 

 parts is appearing midway between the two chromatin masses. Draw. 



7. Completion of mitosis. The constriction deepens dividing the cell into 

 two cells, the chromatin mass resolves itself into the ordinary nuclear structure, 



