Method and a device for the optical in ovo sex determination of fertilized and incubated birds' eggs. The method includes monitoring formation of at least one identifiable blood vessel, creating a hole in the shell of the egg, finding and irradiating the blood vessel with at least one laser beam source emitting an excitation wavelength, recording backscatter radiation of the irradiated blood vessel and evaluating backscatter radiation from recorded spectral intensity of fluorescence radiation in a spectral range redshifted to an excitation wavelength. Sex-specific properties of male and female blood are contained in the intensity and spectral profile of the recorded fluorescence radiation. where intensity levels for male blood has an evaluable value distinct from that of female blood. Further, determining the sex of the bird egg from difference values of the fluorescence intensity and displaying the sex determined for the embryo in the egg.
