So in these equations you're using the Equation 2 form where 
is 2, θ is 1, n is 9.65, τ is 2, and γ is 1?
is 2, θ is 1, n is 9.65, τ is 2, and γ is 1?If the density of the cells starts off at P(t) = 0 (as is the case based on your history function) why do you expect any cells to spontaneously come into existence? If I changed your history and the time over which I solved the system (to fit in the time restrictions of MATLAB Answers) I got a non-constant answer. Whether or not this is correct is something for you to determine.
If you're going to generalize this I'd define the parameters as constant values in ddefun (or have ddefun accept them as additional parameters) to make it easier to modify the equations (simply by changing the parameter values, rather than changing the equations.) I did this for two of the parameters, n and tau.
sol = ddesd(@ddefun,@delay,@history,[0 30]); % Changed from 30000 to 30
t = sol.x;
x = sol.y;
plot(t,x);
function dxdt = ddefun(t,x,Z)
n = 9.65;
dxdt = 2*Z/(1 + Z^n) - x;
end
function d = delay(t,x)
tau = 2;
d = t - tau;
end
function v = history(t)
v = 0.5; % Changed from 0 to 0.5
end
