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 Cinin) 
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- 5. 9 an Amosba—a unicel: 
pied by the linin-thread. . lulaf organism consisting of a 
There are in most cases one ae otepant ee a 
or more nucleoli present, oles. She Be SUD ME Cae 
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 preéx- 
isting nucleus. There is no evidence that it can ever 
arise de novo. 
Nuclear division is of two leinds, 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 
