[Embryologic problems raised by proboscis lateralis].

Organization is conserved in proboscis lateralis and in this respect it represents a half-nose and not a nasolacrymal canal. It is characterized by mainly skeletal deficiency and a paramedian lateral localization. Associated alterations involved olfactory and ocular structures, often with a disorder of closure of the colobomic slit, but overall neuroblast development is not disturbed. Effects on the subjacent facial regions are variable and remain lateral, the overall situation in relation to Tessier's fissure tending to clarify their significance. Comparison of the malformation with the different degrees of cyclopia and with the bifid nose confirms its origin from a particular form of embryogenic process. Significance of participation of the olfactory placodes and the neighboring ectomesenchyme is discussed, together with the modalities of this participation. The latent bilateral nature of the malformation must be recognized together with its embryogenic consequences.