4 EMBRYOLCXiY OF INSECTS AND MYRIAPODS 



Cell Division. — Cells multiply by direct, or indirect division. By 

 direct division (amitosis) the cell including the nucleus is merely pinched 

 in two. This process is characteristic of aging cells. Indirect division 

 (mitosis), the usual method of nuclear division, is a series of complex 

 processes which may be arranged in several phases (Fig. 2). In the 

 resting stage {A) of the cell the so-called "chromatin granules," composed 

 of a dark staining substance, are apparently scattered through the nucleus. 

 When about to divide, this chromatin material takes on the form of 

 threads or a long single strand, or spireme {B), which later separates into 



Fig. 2.— 



E F ^ 



Mitosis. A, resting stage. B-D, prophase. E, metaphase. F, anaphase, 

 telophase. H, late telophase returning to resting stage. 



several pieces. This is the prophase stage. The nuclear wall breaks 

 down when the spireme segments into a number of bodies called "chromo- 

 somes" {C,D). These bodies become arranged in a plate at the equator 

 of a spindle halfway between the centrosomes. In the next stage — the 

 metaphase — the chromosomes split in such a manner that each of their 

 parts contains an approximately equal amount of chromatin {E). In 

 the next, or anaphase stage, the chromosomes formed after splitting 

 appear to be drawn along the spindle fibers {F) to the centrosomes. 

 Every chromosome present at the end of the prophase sends half its 

 chromatin to either end of the spindle. The next is a reconstruction 

 stage, or the telophase, wherein the nuclei return to the resting condition 

 {G). The chromatin apparently again becomes scattered through the 

 nucleus ; a new nuclear wall is formed ; and the cell itself constricts between 

 the nuclei, producing two daughter cells (Fig. 2H). Growth in all animal 



