58 



FUNDAMENTALS OF CYTOLOGY 



Comparatively little is well known concerning the particular factors 

 responsible for the onset of cell division. Since the mitotic changes are 

 so conspicuous and precede cytokinesis, it is often assumed that cell 

 division begins with the nucleus, all othei- changes being a consequence of 



Fig. 38. — Mitosis in embryonic cells of whitefish: prophase, metaphase, and two stages of 

 anaphase. {Courtesy of General Biological Supply House, Inc., Chicago.) 



its behavior. In certain meristematic cells with large vacuoles it has 

 been shown, however, that the cytoplasm forms a sort of diaphragm, or 

 phragmosome, across the cell at the plane of future cytokinesis before the 

 nucleus, about to divide, becomes oriented with respect to this plane 



