Continuation of life starts from cells either those of the general body or the sex cells (gametes).
Virchow (1821-1902) a proponent of cell theory is given the credit for the phrase Omnis cellula de cellula, or cells arise from pre-existing cells, indicates the importance of cell division in the creation of new cells.

In 1852 a German scientist, Robert Remak, published his observations on cell division, based on his observations of embryos. This was one of the first attempts to understand the mechanism of cell division. He stated that binary fission of cells was the means of reproduction of animal cells. What happens during cell division could only be understood better when scientists came to know what is present inside the nucleus of the cell.
In 1879 Walther Flemming (1843-1905) examined many kinds of animal and plant cells and selected those that showed division. He reported from his observations of such cells that there were string like structures in the nucleus which split longitudinally during cell division. He named such a process of division as mitosis (mitos- means fine threads) as the dividing structures resembled threads. He made a meticulous observation and made sketches and observed that there were a sequence of events in the process of division. A decade later these thread like structures were named as chromosomes (coloured bodies) as repeatedly in efforts to see them scientists were trying to use dyes to stain the nucleus and found that these structures were stained most often. His most important discovery was chromosomes appear double in nature.
Wilhelm Roux (1850-1924) proposed that chromosomes carried a different set of heritable elements and longitudinal splitting observed by Flemming, ensured the equal division of these elements. Combined with the rediscovery of Gregor Mendel's 1866 paper on heritable elements in peas, these results highlighted the central role of the chromosomes in carrying heritable material (or genetic material). In cell division the cell divides into two halves with equal number of chromosomes which are similar to parent cell and are diploid in nature.
But the chromosomes number always remained the same. Biologists.