24 THE CHROMOSOMES 



Second Stage of Anaphase 



When the autonomous movement of the chromatids 

 has come to an end a remarkable change in shape 

 takes place in the spindle (Fig. 2e). Its middle region 

 between the two groups of spindle attachments 

 undergoes elongation so as to complete the separation 

 of the two sets of chromatids (which must now be 

 called chromosomes). The growth and elongation 

 of the middle region of the spindle to form a stem- 

 body is apparently a universal feature of mitosis ^ ; 

 unfortunately we have no idea what it is due to. 

 The stem-body is clearly a solid gel like the rest of 

 the spindle ; under abnormal conditions (e.g. in 

 some hybrids and in cells cultured in hypertonic 

 solutions) it may go on growing until the spindle is 

 forced by lack of space to curl round in the cell 

 (Fig. 3rf). Usually the stem-body shows con- 

 spicuous longitudinal striations which are probably 

 remnants of the divisions between the original 

 spindle elements. 



Telophase 



The two groups of ' daughter chromosomes ' never 

 actually reach the poles of the spindle, although as a 

 result of the elongation of the stem-body they may 

 travel farther apart than the original distance be- 

 tween the poles of the metaphase spindle. When 

 the cell has reached the stage represented by Fig. 2e 

 the polar caps of the spindle disappear by a process 

 of gel-solution ; the stem-body, on the other hand, 

 frequently persists for a long time, even after cell 

 division has been completed (Fig. 2/). As the polar 

 caps of the spindle are destroyed a new nuclear 

 membrane is formed round each of the telophase 

 groups of chromosomes. The details of the process 

 whereby the cytoplasm becomes divided into two 

 daughter cells are outside the scope of this book. 



The changes which take place inside the nuclear 



