62 



THE RESTING AND DIVIDING CELL 



Photomicrograph of a section of a growing 

 root tip of an onion. How many different stages 

 of cell division can you identify ? 



The spireme breaks up into a number of parts called chromosomes. 

 Each plant or animal cell seems to have a definite number of these. 



In the cells of human beings, 

 the spireme breaks up into 

 forty-eight chromosomes. In 

 the onion cell, there are sixteen. 

 In a certain little fruit fly, 

 there are only eight. These 

 numbers are always constant 

 for a particular species. Four 

 chromosomes are shown in the 

 diagrams (page 61) illustrat- 

 ing mitosis of an animal cell. 

 The asters separate more 

 and more widely causing the 

 fibers between them to be- 

 come longer and to assume 

 a spindlelike shape. Finally they reach the opposite sides of 

 the nucleus. The chromosomes are now drawn into the central 

 part of the spindle forming a ring around it, in a plane called 

 the equatorial plate. All of thes3 changes are a part of the 

 prophase stage (beginning) of mitosis. The prophase ends with 

 the chromosomes arranged on the spindle in the equatorial plate. 

 Each chromosome now splits lengthwise into exactly two similar 

 parts. The actual splitting of the chromosomes may occur before 

 the arrangement at the equatorial plate. Division of the chromo- 

 somes has been accomplished. This is the metaphase (middle) 

 stage of mitosis. 



The split chromosomes begin to separate and move toward 

 the centrosomes or the poles. x\s they approach their poles the 

 chromosomes los3 their regularity of outline, and upon reaching 

 the poles each group becomes converted into two new nuclei. 

 This is the anaphase (approaching the end) stage of mitosis. 



