Appearance of a Purkinje shift in the developing retina of the chick.

The development of rod and cone function was studied in late embryonic and posthatch chicks by measuring the spectral sensitivity of the electroretinogram (ERG) of opened eyecups to 100-msec flashes and 25-Hz stimuli. At the time of hatching the ERG is dominated by cone activity: the response-energy curves for different wavelengths are parallel, and the spectral sensitivity of the dark-adapted eye is maximal near 580 nm. During the first week after hatching, rods make a progressively greater contribution to the ERG: the response-energy functions cease to be parallel, and the spectral sensitivity maximum to dim flashes moves to about 510 nm while the maximum for brighter lights remains at 580 nm. Only the cones are able to follow the 25-Hz stimulus. Spectral sensitivity of the eye to 25-Hz stimuli, measured on red- or orange-adapting backgrounds, reveals the presence at the time of hatching of two photopic processes with lambda max at 415 nm and 580 nm, with evidence for a third in the region 470-510 nm.