Fig. 11.

P and Q in relation to proliferative fate. Theupper bar of the double abscissa marks the elapsed integer cell cycles CC_1–11 of the dorsomedial murine cerebral PVE; the lower bar of the abscissa marks the elapse of time in Embryonic Days. The ordinate provides a calculation, on the basis of the data shown in Figure 7 and the binomial theorem, for the proportion of total mitotic divisions at each integer cycle that will give rise only to P cells (P+P symmetric proliferative fate), only to Q cells (Q+Q symmetric proliferative fate), or to P + Q cells (P+Q asymmetric proliferative fate). For example, if Q = 0.3 and P = 0.7, there will be Q² = (0.3)² = 0.09 and P² = (0.7)² = 0.49 symmetric divisions and 2 × P × Q = 2 × 0.3 × 0.7 = 0.42 asymmetric divisions. All three types of mitoses could exist at all times, but the sum of their proportions will be equal to 1.0. Early in the neuronogenetic period, the P+P symmetric cell divisions should predominate. Late in the neuronogenetic interval, the Q+Q symmetric cell divisions should predominate. The proportion of asymmetric cell divisions should reach its maximum when P = Q = 0.5, i.e., at approximately CC₈, at which time the proportion of Q+Q- and P+P-type symmetric cell divisions should be equal.