CELL-CONTENTS AND FORMS OF CELLS. 135 



as the ground substance of the protoplast and which is now com- 

 monly referred to as the CYTOPLASM (see Frontispiece). These 

 differentiated bodies and the cytoplasm are intimately associated 

 and interdependent. The nucleus and cytoplasm are present in 

 all living cells, and it is through their special activities that cell 



j*F*';t* - 

 fr-^:., .r --i 



FIG. 85. Successive stages in nuclear and cell division, n, nucleolus; c, centre-spheres 

 s, chromosomes; sp, spindle fibers; A, B, C, division of chromosomes, i, cell with nucleus 

 containing nucleolus (n), and two centrospheres (c) ; 2, showing separation of nucleus 

 into distinct chromosomes (s) and the centrospheres at either pole of the nucleus; 3, forma- 

 tion of spindle fibers (sp) ; 4, longitudinal division of chromosomes; 5, division of the cen- 

 trospheres; 6, 7, 8, further stages in the development of the daughter nuclei; 9, formation 

 of cell- wall whica is completed in 10 giving rise to two new cells. After Strasburger. 



division takes place. When, in addition, plastids are present, con- 

 structive metabolism takes place, whereby complex substances are 

 formed from simpler ones. 



Besides the nucleus and plastids other protoplasmic structures 

 are sometimes found embedded in the cytoplasm. These are the 

 CENTROSPHERES (Fig. 85, c) , small spherical bodies that are 



