  Our main result shows that the mass $2\pi$ is critical for the minimal
Keller-Segel system \begin{align}\label{prob:abstract}\tag{$\star$}
\begin{cases} u_t = \Delta u - \nabla \cdot (u \nabla v), \\ v_t = \Delta v - v
+ u, \end{cases} \end{align} considered in a quarter disc $\Omega = \{\,(x_1,
x_2) \in \mathbb R : x_1 > 0, x_2 > 0, x_1^2 + x_2^2 < R^2\,\}$, $R > 0$, in
the following sense: For all reasonably smooth nonnegative initial data $u_0,
v_0$ with $\int_\Omega u_0 < 2\pi$, there exists a global classical solution to
the Neumann initial boundary value problem associated to \eqref{prob:abstract},
while for all $m > 2 \pi$ there exist nonnegative initial data $u_0, v_0$ with
$\int_\Omega u_0 = m$ so that the corresponding classical solution of this
problem blows up in finite time. At the same time, this gives an example of
boundary blow-up in \eqref{prob:abstract}. Up to now, precise values of
critical masses had been observed in spaces of radially symmetric functions or
for parabolic-elliptic simplifications of \eqref{prob:abstract} only.
