40 LABORATORY STUDIES IN ZOOLOGY 



just described, a splitting of the chromatin threads has occurred. 

 These split threads or chromosomes appear to shorten and 

 thicken. The nuclear membrane disappears leaving the chromo- 

 somes free in the cytoplasm. The chromosomes now appear 

 in pairs, due to splitting, and become arranged upon the spindle 

 in a plane midway between the two centrosomes. This plane is 

 referred to as the equatorial plane. The series of events culminat- 

 ing in this manner is said to constitute a stage in mitosis known 

 as the prophase or the preparation stage for actual division. 



Metaphase. — Following late prophase there appears to be a 

 short period during which the chromosomes are arranged in 

 the equatorial plane. It is a state of balance. If the chromo- 

 somes have not already split they do so at this time. 



Find a cell cut across the middle of the spindle (in an equatorial 

 plane) and showing the four split chromosomes free in the 

 cytoplasm. Centrosomes and astral rays will not show in such 

 a section. Draw. 



Locate a cell which is cut through the axis of the spindle (in a 

 meridional plane). This should show centrosomes, astral ray, 

 and spindle with the band of chromosomes across the middle of 

 the mitotic figure. Draw. 



Anaphase. — One of each pair of the split chromosomes moves 

 toward its respective centrosome or aster. In this migration 

 the ends of the chromosomes always point toward the axis of the 

 spindle, so that the cell contains two groups of chromosomes, 

 each group looking somewhat like an open umbrella with the 

 delicate threads of the spindle stretching between them. Draw. 



Telophase. — The chromosomes approach the asters, where 

 each group condenses into a mass in which the identity of the 

 chromosomes is lost. A constriction in the cytoplasm gradually 

 deepens and divides the cell in two. Draw. The chromatin 

 mass of each daughter cell reorganizes a nucleus with a nuclear 

 membrane. Two daughter cells, believed to be exactly like the 

 original mother cell and like one another, have thus been 

 produced. 



