At the same developmental stage at which a photoreceptor potential can first be recorded in the chicken's retina a visually evoked response can be elicited in its optic tectum. Retina and optic tectum, therefore, seem to start functioning simultaneously. This paper discusses experimental investigations into how the various components of the chicken visual system develop. The investigation was carried out in three steps. With morphological techniques the growth and differentiation processes of retinal ganglion cells compared to that of receptor cells were observed and a quantitative study of the development of optic nerve fibres was made. With physiological techniques the functional properties of retinal ganglion cells and their axons were tested at successive stages of development. A correlation between the morphogenesis and the physiogenesis of this system was then carried out. A model to explain the growth in fibre diameter with time was constructed. The relation between conduction velocity and fibre diameter was studied and was found to be different from that in the adult. For myelinated fibres a theoretical explanation for the relationship in the developing chicken could be found, and it could also be shown that as time progresses such a relationship passes over to that found in the adult. The final link in the construction of the chain of elements forming the visual system was found to be the late maturation of the receptor outer segments in the retina.