72 



PRINCIPLES OF ANIMAL BIOLOGY 



shortens and the parts of the skein become further separated. In this 

 condition the thread is called a loose skein or loose spireme (Fig. 37). 



Meanwhile other changes in the cell have been taking place. The 

 nuclear membrane has usually disappeared or is in process of disinte- 



FiG. 35. — Resting cell. The centro- 

 some has divided preparatory to mitosis, 

 and the nucleolus is present. 



Fig. 36. — Formation of the close 

 spireme and development of the amphi- 

 aster in a cell about to divide. Figures 

 36-40, inclusive, illustrate prophases. 



gration. If a centrosome surrounded by a centrosphere was present in 

 the cell it has already divided, and about each of the two resulting cen- 

 trosomes a figure made up of radiating lines (the aster) has appeared. 



Fig. 37. — Loose spireme in a dividing 

 cell. The spindle is enlarging. 



Fig. 38. — Dividing cell in which chro- 

 mosomes have been formed by the seg- 

 mentation of the spireme. The nuclear 

 membrane and nucleolus are degenerat- 

 ing and the astral rays are entering the 

 nucleus. 



There have also appeared other lines arranged in the form of a spindle 

 extending from the region of one centrosome to that of the other. The 

 whole structure (spindle and asters) is called the aniphiaster. Since 



