INTRODUCriOX. 3 



the cell-wall, being separated only by plates of proto|)lasm. 

 When all vacuoles fuse into one the cytoi)lasm 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. iSS). 



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 si)indle- 

 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 licpiid dyes, while 

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



Ftc. 2. — A part of the s.anie cell as in ^z. i, but older, with the nucleus beRinnina: to 

 divide, 'i'he dark thread in A, separated into pieces in B. represents the chroma- 

 tin of the nucleus dccph' 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. 



