246 RECENT CYTOLOGY 
Each of the cells hitherto referred to possesses a single 
nucleus, which is usually a more or less spherical body 
occupying a central position within the cell. Nuclei, 
like the cells which contain them, arise only by the 
division of pre-existing nuclei. Thus the history of 
the nuclei is in every way similar to the history of the 
cells, of which they constitute so important a part. 
In fertilization the nuclei of the conjugating cells or 
gametes fuse together to form the single nucleus of 
the fertilized ovum, and every division of this cell, as 
well as of its cell-progeny, is preceded by a division 
of the nucleus into two similar portions. 
We may forthwith concentrate our attention upon 
the nucleus as being that part of the cell which is of 
primary importance from the point of view of heredity, 
for it is now generally recognised that the nucleus 
is the part of the cell in which hereditary features 
are in some way carried. And we may next consider 
a little more closely the structure of the nucleus as 
seen under high powers of the microscope. : 
In what is somewhat improperly called its resting 
condition—a condition which is characteristic of nuclei 
at all times when they are not actually undergoing 
division, or preparing for that process—the nucleus may 
be seen to be bounded by a more or less definite nuclear 
membrane. The internal structure of such a nucleus is 
described as reticular—that is to say, at least two 
different substances are differentiated within the 
nucleus, one of them forming a reticulated meshwork, 
the interspaces of which are occupied by the other 
(Fig. 19, p. 232). 
