6 THE BREEDING OF ANIMALS 



phases which result from the varying degrees of activity 

 present in the nuclear substance. One phase may be 

 designated as the vegetative or quiescent stage and the 

 other the active stage which is characteristic of that 

 period in the development of the nucleus when the many 

 complicated and significant changes occur which result 

 in cell division and in reproduction. 



The typical nucleus during the vegetative stage pos- 

 sesses certain distinct structural forms which are con- 

 cerned in the many important nuclear activities : (1) 

 The nuclear wall which encloses the nucleus and dif- 

 ferentiates the nucleus from the cell-body; (2) The 

 reticulum, which is the primary factor in nuclear activities, 

 and appearing as an irregular network. The reticulum 

 in turn comprises two structures, the linin and the chro- 

 matin. The latter is undoubtedly the most fundamentally 

 important organic substance concerned with the growth, 

 development and inheritance of plants and animals. 

 It is apparently the only or chief material which is trans- 

 mitted from the parent cell to the new or daughter 'cell 

 by division, and from it all nuclear substance may be re- 

 formed. The word chromatin is given to this material 

 because it becomes deeply stained upon the addition of 

 certain well-known reagents. The chromatin may appear 

 in the cell in scattered granules, varying in size and form, 

 or in a single deeply staining mass, but more often the 

 arrangement of the chromatin in the nucleus resembles 

 a network which is closely associated with the clearly 

 differentiated linin. (3) The nucleoli are generally but 

 not always present, and their nature and functions are 

 not well understood. By some authorities the nucleoli 

 are the by-products of the activities going on in the 

 nucleus. (4) The ground substance is a fluid filling 



