CELL-NUCLEUS—FERTILISATION 227 
The nucleus is perhaps the most important 
organ of the cell. There are strong grounds 
for believing that it is largely concerned in the 
determination of those hereditary qualities 
which distinguish one species from another; 
and we are also well aware of its great import- 
ance in governing the chemical changes which 
proceed within the protoplasm. 
The nucleus consists, essentially, of a 
variety of substances, more or less gelatinous 
in consistency. These, together with more 
fluid constituents, are contained within a 
membrane, and are thus sharply delimited 
from the surrounding cytoplasm. The con- 
tents of the nucleus are not homogeneous. 
One or more spherical bodies, the nucleoli, 
may often be seen inside it. These, although 
often very prominent, are of subordinate im- 
portance, inasmuch as they chiefly represent 
reserves of material to be drawn on at periods 
when the nucleus is undergoing division. The 
more solid gelatinous matrix (linin) contains 
the most important nuclear constituents. A 
more or less finely divided substance distri- 
buted in the gelatinous matrix often gives the 
nucleus a rather granular appearance. Stains 
of various kinds render this much more 
evident, and the stainable particles are often 
known as chromatin. 
When the nucleus is about to divide, strik- 
ing rearrangements are observed to take place 
within it. The gelatinous linin, in which the 
chromatin is diffused, contracts, and at the 
