THE ORGANIZATION OF THE INDIVIDUAL 21 



partly attributed to a constriction of the spindle in the region of its 

 equatorial plate. 



It is during anaphase that individual chromosomes show their charac- 

 teristic "J" or "V" shapes. The former shape is associated with a cen- 

 tromere near the end of the^ chromatid, the latter with a centromere 

 near the center. The free ends of the J or V are swept back as the 

 chromatid is drawn through the viscid material of the spindle. 



Telophase. The telophase may be thought of as prophase in reverse. 

 The two groups of chromatids (which now constitute the chromosomes 

 of the daughter nuclei) have come to lie near the opposite poles of the 

 spindle. The daughter chromosomes now gradually uncoil and begin to 

 lose the property of taking a clear-cut stain. The spindle disappears, and 

 a nuclear membrane forms about each group of chromosomes. Any 

 nucleolus or nucleoli that disappeared at prophase reappear. The cyto- 

 plasm now divides, usually in the plane occupied by the equatorial plate 

 of the spindle, to form two complete and separate daughter cells. 



The foregoing account has been kept as brief as possible. Many details 

 of interest to the cytologist have been omitted, and little notice has been 

 given to the variations in other details that would be found if one com- 

 pared the mitotic processes of a number of different kinds of cells. We 

 have attempted to describe the essential events and consequences of 

 mitosis that are of marked importance in nearly all considerations of 

 biological processes. The following points should be particularly noted: 



1. Mitosis results in a precisely equal division of each chromosome, 

 so that the daughter cells receive nuclei that are qualitatively and quanti- 

 tatively identical. 



2. The chromosomes are seen to be double (paired chromatids) at the 

 beginning of prophase, but single when last discernible in telophase. The 

 doubling process evidently takes place during the resting stage, as should 

 be expected theoretically from the necessity of each chromatid duplicating 

 itself by metabolic synthesis of new chromatin material. Unfortunately, in 

 a great number of cells the chromosomes of the resting cell cannot be 

 made visible by the techniques of the microscopist, probably because they 

 imbibe large quantities of water and so change from the condensed condi- 

 tion which they exhibit during mitosis. There is, however, ample reason 

 to believe that they remain intact throughout the resting stage. 



3. Our diagram shows six chromosomes. If they are compared, it will 

 be noted that the six include three different sizes of chromosomes, each 

 size represented twice. The members of each pair are to be thought of as 

 being not only alike in size but in all other respects as well. If we should 

 examine the cellular structure of a wide variety of plants and animals, we 

 would find (with certain exceptions to be noted later) that all the cells 



