INTROD UCTION. 3 



the cell-wall, being separated only by plates of protoplasm. 

 When all vacuoles fuse into one the cytoplasm is crowded as 

 a thin layer against the wall, with sometimes strands of it 

 crossing the vacuole as the remnants of the plates at an 

 earlier stage (fig. 188). 



6. Nucleus. — The nucleus varies much in shape. In cells 

 whose diameters are nearly equal, it is generally spherical 

 or ovoid, but in elongated cells it may become spindle- 

 shaped or cylindric. It is surrounded by a very delicate 

 membrane, and is composed of two sorts of substances, one 

 of which can be readily stained by certain liquid dyes, while 

 the other usually remains uncolored (fig. 2). The nucleus 



Fig. 2. — A part of the same cell as in fig-. 1, but older, with the nucleus beginning to 

 divide. The dark thread in A, separated into pieces in B, represents the chroma- 

 tin of the nucleus deeply stained, the rest of the nuclear material being unstained. 

 a, centrospheres. Magnified 600 diam. — After Guignard. 



may divide into two, a regular succession of changes in the 

 arrangement of the materials composing it characterizing this 

 process, which is commonly followed by the formation of a 

 partition- wall separating the cell into two parts, each con- 

 taining one of the daughter-nuclei. 



