Olfactory placode is the anlage of the olfactory epithelium and known to produce various kinds of cells other than olfactory receptor neurons. The earliest cells that migrate from the olfactory placode at embryonic day 2 (E2) (Hamberger and Hamilton stage 14) are immunoreactive for polysialylated NCAM (PSA-NCAM) and HuC/D (neuronal markers) in chick embryos. Then these cells form the cellular cord between the olfactory epithelium and the rostral forebrain (anlage of the olfactory bulb). Along the cellular cord, the first olfactory nerve axons grow and GnRH neurons start to migrate on E3 (stage 21). The present study examined development of the cellular cord in whole-mount specimens by immunohistochemistry. Before formation of the rostral-oriented single cellular cord, thick or thin branches of PSA-NCAM positive cells were found to be oriented to dorsal or caudal direction. To elucidate the origin of cells detected in dorsal and caudal branches a GFP vector was introduced to epithelial cells of the olfactory placode by an electroporator. About 40 hours later, electroporated embryos were fixed and GFP-labeled cells were detected after immunohistochemical staining. GFP-labeled cells were found in the olfactory epithelium and PSA-NCAM-immunoreactive cellular cord between the olfactory epithelium and the rostral forebrain and also in the dorsally or caudally oriented branches of the cellular cord. These results indicate that dorsal and caudal branches of cellular cord originate from the olfactory placode. As evidenced by immunofluorescent staining for PSA-NCAM, HuC/D or laminin, a network-like structure with long horizontal PSA-NCAM-positive processes and HuC/D positive cell bodies were found over the telencephalon. The network seemed to locate in the marginal zone beneath the laminin-positive basement membrane and to be composed of neurons that were generated at the earliest stage and may be present in the layer corresponding to Cajar-Retzius cells in mammals. Within the network, a few GFP, PSA-NCAM and HuC/D expressing cells were detected. These results indicate the possible involvement of the olfactory placode-derived neurons to development of the telencephalon.