Light alterated changes in the embroynic age versus incubation age of White Leghorn embryos

White Leghorn eggs were incubated in the dark or under two 20-Watt cool white flourescent tubes. The embryos were collected and staged according to somite number and other anatomical observations. Embryos from lighted eggs were significantly more advanced in embryonic age within 24 hours than embryos from the dark. By the end of 10 days no further acceleration of development by light was significant.