AbstractFour developmental stages of sea urchin embryos were labeled with colloidal gold in an attempt to elucidate the intracellular trafficking patterns within the cells that produce the glycoprotein matrix of the embryonic spicule. The primary mesenchyme cells (PMCs) form a syncytium and secrete an organic matrix on which calcium carbonate is laid down to form an endoskeletal spicule. The organic matrix has been isolated and characterized as glycoprotein consisting of four major bands. Polyclonal antibodies to these glycoproteins were used to label embryos from the mesenchyme blastula, early gastrula, late gastrula, and plutei stages of development. The label is concentrated in the Golgi complex and associated vesicles, in secretory vesicles, and in the organic matrix. The density of the labeling increases as development proceeds.