THE NUCLEUS 229 



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). 



