(a) When an initial explant is made of human cells, some cells die and others (mainly fibroblasts) start to grow; overall the growth rate increases (phase I). If the remaining cells are continually diluted, the cell strain grows at a constant rate for about 50 cell generations (phase II), after which the growth rate falls rapidly. During the ensuing period of increasing cell death (phase III), all the cells in the culture eventually die. (b) In a culture prepared from mouse or other rodent embryo cells, there is initial cell death coupled with the emergence of healthy growing cells. As these are diluted and allowed to continue growth, they soon begin to lose growth potential and most cells die (the culture goes into crisis). Very rare cells do not die but continue growing until their progeny overgrow the culture. These cells constitute a cell line, which will grow forever if it is appropriately diluted and fed with nutrients: the cells are immortal.