44 THE ELEMENTS OF STRUCTURE. 



the cell substance, but it may shift its position from one 

 part of the cell to another. It has a definite margin, but 

 this may be lost, e.g. before cell division begins. Internally, 

 it is anything but homogeneous ; at any rate, homogeneous 

 nuclei are rare. Usually there is a network of fine, strongly 

 stainable (chromatin) strands, with less stainable (achro- 

 matin) substance in the meshes. But in other cells, or at 

 another time in the same cell, the nucleus is seen to 

 contain a coiled (chromatin) thread, or a number of chro- 

 matin loops (Fig. 19). Weismann and others believe that 

 these chromatin elements or chromosomes are made up 

 of hypothetical bodies whose properties are supposed to 

 determine the nature of an organism and its life. Many 

 nuclei also contain one or more little round bodies or 

 nucleoli, apparently of less importance. 

 The term is applied somewhat vaguely 

 to little aggregations of chromatin, and 

 more properly to vacuole-like bodies, 

 in which some believe that the waste 

 products of the nucleus are col- 

 lected. Within the nucleolus an 

 " endo-nucleolus " has been discovered. 

 Though the nuclei of different cells 

 Fig. 19. — Structure differ in details, there is a fundamental 

 of the cell.— After sameness, both of structure and activity, 

 arnoy ' throughout the world of cells. 



^ctif^ot/p™: (') As to the centrosomes, it may be 

 topiasmic reticulum. noted that when an animal cell divides, 

 these bodies play an important part. 

 The chromatin elements of the nucleus are divided, and 

 separate to form the two daughter nuclei. In this separation 

 extremely fine " archoplasmic " threads pass from the 

 centrosomes to the chromosomes. These centrosomes are 

 therefore regarded as " division organs," or as " dynamic 

 centres." They also occur, in most cases singly, in resting 

 cells, and it seems likely that they are present in most 

 animal cells, at least in those which retain the power of 

 division. 



(d) As to the cell wall, it seemed of much moment to the 

 earlier histologists, who often spoke of cells as little bags or 

 boxes. It is, however, the least important part of the cell. 



