200 SCIENCE FROM AN EASY CHAIR 



of the structure of the protoplasmic corpuscle which we 

 call a " cell," and show that the universal presence of the 

 nucleus in every " cell " is due to the fact that it plays 

 the most important part in the life of the cell. It is the 

 seat of control, and contains substances in virtue of which 

 the changes which constitute growth and form-production 

 take place, and in the absence of which the rest of the 

 protoplasm cannot " carry on," although for a time it lives ; 

 that is to say, remains chemically undecomposed, and 

 shows active movement. At the same time, we must 

 not underrate the importance of the general protoplasm, 

 without the presence of some of which the nucleus cannot 

 do its work, nor even exist. It is no wonder, then, that 

 when a cell divides, there are curious and elaborate 

 proceedings in the nucleus, by which each daughter cell 

 gets its due half of the all-important nuclear substance. 



When a cell divides the fission or splitting of the 

 cell is preceded by peculiar changes in the nucleus. 

 There is a material in the nucleus of every cell of those 

 which are simple animalcules, as well as of those which 

 are germ-cells and sperm-cells, and of those which form, 

 heaped up in enormous numbers, the living substance of 

 larger animals and plants a material which is an 

 elaborated sort of proteid (see p. 185) and stains strongly 

 with carmine, logwood, and such dyes, and is called 

 " chromatin." It exists often in the shape of minute 

 granules and filaments (Fig 42^), but always takes on, 

 sooner or later, the form of an irregularly undulated 

 thread or threads. When the cell is about to divide into 

 two as all growing and active cells do the thread 

 arranges itself like a zigzagging girdle around the 

 equator of the globular nucleus (Fig. 42<5). The margin 

 of the nucleus then seems to melt away into the general 

 protoplasm, and the zigzag bits of the stain able thread 

 break from each other, forming a ring-like group of 



