INTRODUCTION 5 



The nucleus shows a complicated structure, being com- 

 posed of a very much twisted filament, more or less 

 fused together at certain points, of a substance (limn) 

 which does not readily take up the ordinary stains used 

 in histological studies. In this linin-thread are numer- 

 ous granules of a peculiar 

 substance, chromatin, char- 

 acterized by its avidity for 

 various nuclear stains. A 

 colorless fluid, or semi-fluid 

 substance fills the space of 

 the nuclear cavity not occu- 



^ . FIG. 2. An Amoeba a umcel- 



pied by the lllim-thread. lular organism consisting of a 



rm . nucleated mass of protoplasm 



1 here are in most cases one without a cell-wall, n, nucleus; 

 or more nucleoli present, ^ of the contractile vac ~ 

 globular bodies which gen- 

 erally stain strongly, but whose nature is still some- 

 what doubtful. Of the various constituents of the 

 nucleus, the chromatin is probably the most important, 

 and it is likely that in this substance are contained the 

 elements which determine the peculiar properties of 

 the cell, and in the reproductive cells transmit heredi- 

 tary characters. The nucleus is an essential part of 

 the cell, and is always formed by division of a preex- 

 isting nucleus. There is no evidence that it can ever 

 arise de novo. 



Nuclear division is of two kinds, direct and indirect. 

 By the first method a nucleus simply becomes con- 

 stricted and forms two similar nuclei, or the part sep- 

 arated may be smaller than the main part of the nucleus. 

 This form of division is confined to the lower types of 

 plants, or it may occur secondarily in old cells of some 



