Anti-visinin (7G4, 1:500; developed by Constance Cepko, Harvard University, Cambridge, MA) and anti-BrdU (G3G4; 1:100; developed by Stephen J. Kaufman, University of Illinois, Urbana, IL) were obtained from the Developmental Studies Hybridoma Bank (Iowa University, Iowa City, IA). Other antibodies were purchased from commercial sources: polyclonal anti-AP2 (1:500; Santa Cruz Biotechnology, Santa Cruz, CA), monoclonal PSD-95 (7E3-1B8; 1:200; Sigma, St. Louise, MO), polyclonal SNAP-25 (1:200; Sigma), polyclonal antibody against calretinin (1:500; Chemicon, Temecula, CA), polyclonal anti-red opsin (1:200; Chemicon), and polyclonal antibody against viral protein P27 (1:500; Spafas, Norwich, CT). Monoclonal antibody against chick NeuroD (CND2, 1:20) was developed in our laboratory. Standard immunohistochemistry was carried out with fluorophore-conjugated secondary antibodies goat anti-rabbit Alexa Fluor 546 and goat anti-mouse Alexa Fluor 488 (Invitrogen, Carlsbad, CA).