The early ontogeny of the sensory cranial nerve ganglia was studied in chick embryo during the developmental period between 2-5 days. The histochemical aspects of ectodermal placodes contributing to these ganglia were studied using the lectins UEAI, UEAII, WGA SWGA, PNA and SBA. Trials to use the method of carbon labeling as a cytological marker for the ectodermal placodes were shown to be invalid. The histological results provide evidence that the endoderm of the pharyngeal roof share the other known sources in the formation of the head mesenchyme. The cells of the head mesenchyme aggregate at the ganglionic sites and differentiate into ganglionic cells. A topographic relation between the parts of the ganglia and the different sources contributing to them was suggested. Together with the histological criteria of the ectodermal placodes, it was concluded that sialic acid and galactose expression on the cell surface are markers that can be used to delineate the ectodermal placodes. Galactose may be related to stratification of the placodes, while sialic acid may provide the repulsive sources that encourage the process of epithelial-mesenchymal transformation from the placodes. These delineation criteria were lost during and after the third day of development. The lectins histochemistry also shows that N-acetylglucosamines were located inside the cells of the ganglionic primordia and some of the mesenchymal cells surrounding the ganglia. However, this sugar moiety is lost from inside of these cells after the second day of development, its disappearance may be related to the formation of the nerve fibers. Sialic acid and galactose show similar pattern of distribution to that of N-acetylglucosamines, but they disappear from inside of the cells during and after the third day of development. The disappearance of these sugar moieties from the mesenchymal cells may be associated the aggregation of these cells at the ganglionic sites to form the multilaminated capsule around the ganglia. SBA and PNA binding were shown to be a criteria of neuroectodermal cells, therefore these lectins may be used as a neuroectodermal marker of the migrating mesenchymal-like cells. An elementary cusp model was constructed based on the catastropb theory to provide a mathematical model for the embryonic development