AbstractLectin histochemistry was used to examine the expression of cell surface glycoconjugates during secondary neurulation in chick embryos. Fourteen lectins were applied to serial sections of the caudal region of embryos at the various stages of tail bud development. The lectinsBandeiraea simplicifolia, Dolichos biflorusagglutinin,Phaseolus vulgarisleukoagglutinin, soybean agglutinin,Sophora japonicaagglutinin,Ulex europaeusagglutinin and succinylated wheat germ agglutinin (sWGA) showed very light or no binding to the developing medullary cord of the tail bud. With the other lectins, staining occurred throughout the early tail bud and solid medullary cord. During cavitation, however, differential expression of cell surface glycoconjugates by different cell populations was observed. The lectins concanavalin A,Lens culinarisagglutinin,Pisum sativumagglutinin,Phaseolus vulgariserythroagglutinin,Ricinus communisagglutinin and WGA showed basic similarities in the distribution of lectin binding. Of these, the binding pattern of WGA was the most striking. As the medullary cord cells were separating into central mesenchymal and peripheral epithelial populations, WGA bound preferentially to the epithelial cells and the notochord. The lectin PNA, however, became preferentially bound to the mesenchymal cells. Heavy staining by WGA (specific for N‐acetylglucosamine and sialic acid) where sWGA staining (specific for N‐acetylglucosamine only) was faint suggested that WGA binding was due to the presence of sialic acid containing glycoconjuga