GROWTH 91 



child at birth is from three million to four million 

 cubic millimeters, an increase of one billion times. 

 Yet from the first year to the twentieth the ratio of 

 increase is figured at only one to sixteen. 



Since the organism is composed of cells, it is obvious 

 that to accomplish this growth the cells themselves 

 must increase either in size or in number. It was 

 early discovered that both these changes take place, 

 and that the latter seems to be consequent upon the 

 former. We have seen that the nucleus " domi- 

 nates " the rest of the cell, as it were, and that with- 

 out the presence of a small portion of nuclear matter 

 the normal changes of metabolism in the cytoplasm 

 cannot go on. This influence of the nucleus appears 

 to have rather narrow limits, and if the cell gets to 

 be too large, portions of it may get out of the range 

 of the nuclear influence. Sometimes this is avoided 

 by the fragmenting of the nucleus, the parts being 

 distributed about the cell ; but this occurs in only 

 a few kinds of cells. Normally, when the bulk of the 

 cell has increased by growth to the natural limit, the 

 nucleus divides into two halves that move apart and 

 divide the original cell between them, thus making 

 two new cells, separated by a newly formed cell- 

 wall. Occasionally the cell- wall does not form, in 

 which case we have a syncytium resulting. When 

 the two daughter-cells have grown to the size of the 

 original mother-cell, the process is repeated, and so 

 on, the number of cells in a given tissue increasing 

 with ^he growth of the tissue, but the average size 

 of the cells themselves remaining nearly constant. 



