Answer
The curved arrows in the picture show the movement of electrons. $NH_3$ is the Lewis base (nucleophile) and $CH_2O$ is the Lewis acid (electrophile).
Work Step by Step
The lone pairs on Nitrogen attack the C, and then since C now has more than 4 bonds, the bond between the $C$ and $O-$ breaks and the electrons go to O, forming $O^-$. Since $NH_3^-$ donates a pair of electrons it is lewis base and $CH_2O$ accepts a pair so it is lewis acid.
