$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
11.15
\begin{equation} \begin{gathered}\label{equation:mapping-Twarr} \xymatrix@R =50pt@C=50pt{ X_{S} \ar [r] \ar [d] & X_{T} \ar [d] \\ X_{ \operatorname{sk}_{n-1}(S) } \ar [r] & X_{ \operatorname{sk}_{n-1}(T) }. } \end{gathered} \end{equation}