The early chick cornea is composed of an acellular collagenous stroma lined with an anterior epithelium and a posterior endothelium. At stage 27-28 of development (5 1/2 days), this stroma swells so that the cornea is 75-120 mum thick. At the same time, fibroblasts that originate from the neural crest begin to invade this stroma. Using Nomarski light microscopy, we have compared the behavior of moving cells in isolated corneas with the migratory activities of the same cells in artificial collagen lattices and on glass. In situ, fibroblasts have cyclindrical bodies from which extend several thick pseudopodia and/or finer filopodia. Movement is accompanied by activity in these cytoplasmic processes. The flat ruffling lamelli-podia that characterize these cells on glass are not seen in situ, but the general mechanism of cell movement seems to be the same as that observed in vitro: either gross contraction or recoil of the cell body (now pear shaped) into the forward cell process, or more subtle "flowing" of cytoplasm into the forward cell process without immediate loss of the trailing cell process. We filmed collisions between cells in situ and in three-dimensional collagen lattices. These fibroblasts show, in their pair-wise collisions, the classical contact inhibition of movement (CIM) exhibited in vitro even though they lack ruffled borders. On glass these cells multi-layer, showing that, while CIM affects cell movement, fibroblasts can use one another as a substratum. Postmitotic cells show CIM in moving away from each other. Interestingly, dividing cells in situ do not exhibit surface blebbing, but do extend filopodia at telophase. The role of CIM in controlling cell movement in vivo and in vitro is stressed in the discussion.