$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
3.77
\begin{equation} \begin{gathered}\label{equation:coskeleton-vs-fundamental} \xymatrix@R =50pt@C=50pt{ \operatorname{\partial \Delta }^ m \ar [d] \ar [r]^-{\sigma _0} & \operatorname{cosk}^{\circ }_{n}(X) \ar [d]^{q} \\ \Delta ^ m \ar@ {-->}[ur] \ar [r]^-{\overline{\sigma }} & \pi _{\leq n}(X) } \end{gathered} \end{equation}