Sexual differences are found in interphase ectodermic nuclei of chick embryos beginning from 50 hours of incubation: 41% female nuclei show one heterochromatic body larger than others. Such heterochromatic bodies are not seen in interphase nuclei of embryos at 27-47 incubation hours. The cytological differences between sexes are in accord with the caryological ones, and may be utilized in precocious sex diagnosis of embryos in which anatomical or histological differences are not yet detectable.