Figure 21–57 Members of the same species can have dramatically different sizes. the chihuahua weighs  2–5 kilograms, whereas a Great Dane weighs 45–90 kilograms. (courtesy of Deanne Fitzmaurice.)
Figure 21–58 Determinants of organ size.
the proliferation, Death, and Size of cells Determine  Organism Size
The nematode worm C. elegans illustrates the different ways in which size differences can arise. This creature follows an astonishingly precise and predictable developmental program. Each individual of a given sex is generated by almost exactly the same sequences of cell divisions and cell deaths, and consequently has precisely the same number of somatic cells—959 in the adult hermaphrodite (the sex of the majority of these animals)—although the number of germ cells is more variable from worm to worm. The stereotyped development makes it possible to trace somatic cell lineages in exhaustive detail. More than 1000 cell divisions generate 1090 somatic cells during hermaphrodite development, but 131 of these cells undergo apoptotic cell death. Thus, precise regulation of both cell division and cell death determines the final numbers of somatic cells in the worm. In fact, genetic screens in C. elegans identified the first genes responsible for apoptosis and its regulation—thereby revolutionizing our molecular understanding of this form of programmed cell death (discussed in Chapter 18).
The final number of somatic cells in the adult worm is already present at sexual maturity (around three days after fertilization), after which no more somatic cells are generated. Yet the worm continues to grow, doubling in size between sexual maturity and death 2–3 weeks later. This doubling results from somatic cell growth: although the cells no longer divide, they continue to go through rounds of DNA synthesis; this endoreplication of the genome makes the cells polyploid. As in all organisms, the size of a cell is proportional to its ploidy—that is, the number of genome copies that it contains: a doubling of ploidy roughly doubles cell volume. By artificial manipulation of somatic cell ploidy, and thereby somatic cell size, the size of the worm as a whole can be increased or decreased. Thus the worm’s final size is set by a combination of programmed cell divisions and cell deaths, along with regulation of the sizes of individual cells through changes in ploidy.
In plants, as in animals, cell size increases as ploidy increases (Figure 21–59). This effect has been exploited in the agricultural breeding of plants for large size: most of the major fruits and vegetables that we consume are polyploid.
The size of an animal or organ depends on both cell number and cell size—that is, on total cell mass. Remarkably, many animals and organs can somehow assess their total cell mass and regulate it, providing evidence for feedback controls of the sort highlighted earlier in our introductory account of general principles of growth control. In contrast with C. elegans, if cell size is artificially increased or decreased in these cases, cell numbers adjust to maintain a normal total cell mass. This has been beautifully illustrated by experiments done long ago in salamanders, where cell size can be manipulated by altering the animal’s ploidy. As shown in Figure 21–59E, salamanders of different ploidies end up being the same size with very different numbers of cells. The individual cells in a pentaploid salamander,