THE CELL 51 



mitotic, or indirect division, and amitotic, or direct. The 

 latter, while apparently simpler, involving the direct di- 

 vision of nucleus and cytoplasm by a simple pinching into 

 two parts, is yet less common than the former, and no 

 attempt will be made to study the process in this connection. 



Mitosis. In sections of the root tip of the onion, or of 

 the testis of the grasshopper, study the cells which are in 

 the process of division. Find a stage in which the chromatin 

 of the nucleus is forming a long tangled thread, or else a 

 series of densely staining bodies. These bodies produced 

 from the chromatin of the nucleus are called chromosomes. 

 Find a cell in w^hich the nuclear membrane is disappearing. 

 What is the position of the chromosomes? Do you find 

 a spindle made up of delicate fibers to which the chromo- 

 somes are attached? In the cells of some organisms there 

 is a tiny spot, the centrosome, at the end of the spindle and 

 from this centrosome starlike rays, the aster, radiate into 

 the cytoplasm. 



Next examine a stage where the chromosomes are grouped 

 into a mass, or plate, at the center of the spindle. At about 

 this stage each chromosome splits into two parts, and the 

 halves separate and are drawn toward the poles of the spindle. 

 In this position the chromosomes lose their distinctness, 

 form a new nucleus with a membrane, the body of the cell 

 divides into two parts and we have the division of the cell 

 completed. 



The stage of the formation of the chromosomes and their 

 arrangement in the spindle is called the prophase of divi- 

 sion; the splitting of the chromosomes makes up the meta- 

 phase; the separation and the pulling apart of the chromo- 

 somes comprises the anaphase ; the formation of a new nucleus 

 and the division of the cell body is the end or telophase. 



Make a drawing of a cell in each phase of division. 



