THE CELL AND CELL PROLIFERATION. 5 



(spireme thread) arranged in a closed skein — closed spireme (Fig. 3, B). This 

 soon becomes more loosely arranged, the thread at the same time becoming 

 shorter and thicker and frequently broken, forming the open spireme. During 

 the formation of the spireme the nucleolus and nuclear membrane usually 

 disappear, the nucleoplasm thus becoming continuous with the cytoplasm. 

 The spireme now lies with the amphiaster in the general cell protoplasm. 

 The morphological change from reticulum to spireme is apparently accom- 



Fig. 3. — Diagrams of successive stages of mitosis. Wilson. 



A, Resting cell with reticular nucleus and true nucleus; c, two centrioles— the single preceding 



one having divided in anticipation of the division of nucleus and cell body. 



B, Early prophase. Chromatin forming a continuous thread — closed spireme; nucleolus still 



present; a, centrioles surrounded by astral rays and connected by achromatic spindle. 



C, Later prophase. Spireme has segmented to form chromosomes; astral rays and achromatic 



spindle larger and more distinct; nuclear membrane less distinct. 



D, End of prophase; ep, chromosomes arranged in equatorial plane of spindle. 



panied by changes of a chemical nature, as the spireme thread stains much 

 more intensely than do the strands of the reticulum. 



The next step is the transverse division of the spireme thread into a number 

 of segments (Fig. 3, C). These are usually at first rod-shaped, and are 

 known as chromosomes. They may remain rod-shaped or the rods may 

 become bent to form U's or Vs. Some chromosomes are spheroidal. The 

 most remarkable feature of the breaking up of the spireme thread to form 



