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DURING a fine structural analysis of the chick retina, unusual endothelial processes of the choriocapillaris were occasionally found in the matrix of Bruch's membrane. Such processes have not previously been observed in this area. The functional significance of these structures remains as yet unknown. Wolter (1955) observed delicate argentaffin fibre bundles passing through the choroidal matrix and dividing like roots of a tree in the outer layer of Bruch's membrane. It was believed these fibre bundles served as anchors. In this present study, processes of endothelial cells of the choriocapillaris vessels are shown to traverse Bruch's membrane and to come into very close relationship with the retinal pigment epithelium.The purpose of this paper is to describe such endothelial processes of the choriocapillaris and to suggest their possible function. Material and MethodsChicks 5 to 10 days old were anaesthetized with chloroform and the eyes fixed by vascular perfusion through the aorta either with 6-25 per cent. glutaraldehyde (pH 7 2) buffered with 0-067 M phosphate buffer or with 1 per cent. osmium tetroxide (pH 7 4) buffered with 0-067 M phosphate buffer. Following the perfusion, the eyes were enucleated and further fixed in unbuffered 2 per cent. osmium tetroxide at 4°C. for 2 hours. The fixation and subsequent histological procedures have been described previously (Matsusaka, 1967). Ultrathin sections of these eyes were observed with a JEM-6C electron microscope (Japan Electron Optics Laboratory Co., Tokyo, Japan).Observations The endothelium of the choriocapillaris was characterized by a markedly attenuated cytoplasm, about 40 mg thick, on the side of the capillary adjacent to Bruch's membrane (Fig. 1). The several layers of homogeneous and fibrous elements which compose Bruch's membrane (Fig. 2) were less distinct in the current chick material than usually described. The main dense layer (see Fig. 2c, arrow) was discontinuous, but could be recognized near the capillary endothelium. The basal lamina of the choriocapillaris vessels was not clearly demarcated, but that of the retinal pigment epithelium was regular and continuous.