Resting stage, Chromatixi in tangled Spireme divided Chromosomes form 



chromatin in network thread, the spireme into chromosomes ring in middle 



Each chromosome 

 splits lengthwise 



New chromosomes New chromosomes Chromatin in network 



move apart form two tangles of two new tangles 



NUCLEAR CHANGES DURING CELL DIVISION 



The spireme separates into a definite number of chromosomes. Each chromosome 

 splits lengthwise; each half becomes part of the nucleus of one daughter cell. The 

 chromatin becomes exactly divided. The daughter cells have exactly the same num- 

 ber of chromosomes as the parent cell 



Stage in the growth of many-celled species. But the protoplasm of a one- 

 celled plant or animal seems to be able to grow and then divide, without 

 end. Of course no single cell continues to live forever, but the protoplasm 

 — as distinct from the individual — appears to be immortal! 



Nuclear Division^ The sameness of the protoplasm, through all the 

 successive cell-divisions, appears to be related to the behavior of the cell 

 nucleus (see illustration above). We are impressed by the precise division 

 of the chromatin material. It is possible that other parts of the nucleus, and 

 the cytoplasm, also divide in the same precise manner. But of that we cannot 

 be sure, since the substances are for the most part indistinguishable with our 

 present methods of study. 



In multicellular plants and animals cell-division is an essential feature of 

 development, as well as of growth, for at certain stages it results in new kinds 



iSee No. 3, p. 395. 

 368 



