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POPULAR SCIENCE MONTHLY. 



We have already seen how. the conclusion was reached that 

 cells can arise only from pre-existing cells, and that this occurs 

 usually by division. Under favorable circumstances, when a 

 growing cell has reached a certain rather indefinite limit of size, 

 it proceeds to divide into two cells. But each of the new cells 

 must have a nucleus and centrosphere ; and we know that these 

 can only arise from already existing ones, and by division. The 

 process of nuclear division, as before remarked, is usually a very 

 elaborate one, commonly known to students of the cell by the 

 name karyokinesis. Indeed, so fundamentally important does 

 this process appear, that the simpler method sometimes observed 

 is believed by many biologists to have a pathological significance. 

 The first sign of approaching karyokinetic division in a nucleus 

 is the thickening of the threads of the nuclear network. This 

 thickening continues, and at the same time the power of the 

 threads to take up certain staining substances, now much used in 

 their study, rapidly increases. Gradually the network resolves 

 itself into a loose skein or coil, and at last this breaks up into a 

 number, varying much in different species, but pretty constant in 

 the same one, of separate rods or loops. These individualized 

 portions of the original nuclear network have received the name 

 of chromosomes, from their marked capacity for staining. Mean- 

 while the centrosphere, if previously single, has divided, and one 

 of the pair has moved to each of the poles of the nucleus. At 

 least in the higher plants, this division has occurred long before. 



Fig. 1. A plant cell, with its nucleus showing the chromatic network and nucleoli, and with 



a pair of centrospheres. 

 Fig. 2. A plant cell whose nucleus is preparing for division. 

 Fig. 3. A plant cell in an early stage of division, its twelve chromosomes separated. 



The nuclear membrane now disappears and there is formed, per- 

 haps from the homogeneous protoplasm of the centrospheres, or 

 from the substance of the nucleoli, or from both, a spindle-shaped 

 framework of delicate fibers, about whose equator the chromo- 

 somes become arranged in a circle. Then is completed a process 

 which may have begun much earlier, and each chromosome is 

 split longitudinally into two. Of each pair of daughter chromo- 



