Abstract: Sex identification is an important part of poultry production. The general method of vent sexing is time-consuming and labor-intensive. The poultry workers and researchers have been working on in ovo sexing technology for many years, in order to improve the efficiency of gender identification, save energy and labor, and avoid chick culling. This paper summarizes the main methods of in ovo sexing, the latest researches and application, as well as suggestions, and provides reference for the research and promotes application of in ovo sexing technology of poultry. The genetic materials determine the sex of chick embryos. The sex differentiation of gonads gradually becomes clear with the embryo development, and then sex hormones are secreted, resulting in the differences in sex hormones of embryos. With the further development of embryos, male and female embryos will have other differential epigenetic traits, like feather color etc. Thus, Based on the above-mentioned laws of chicken embryo development, the gender of embryos could be identified by detecting the differences of genetic materials, sex hormones, and epigenetic traits between male and female embryos. The in ovo sexing could be achieved by analyzing specific genes on the sex chromosomes with PCR (Polymerase Chain Reaction) or loop-mediated isothermal amplification (Loop-mediated isothermal amplification, LAMP) method. The identification accuracy rate of this method is 100%, but the operation is complicated, and samples with genetic material such as embryonic tissue or blood need to be collected. It is only suitable for scientific research or high-value poultry breeds, and is difficult for industrial application. The gender of embryos could also be identified by measuring the concentration of sex hormones, including estrone, estradiol, testosterone, androstenedione and dihydrotestosterone, etc., with ELISA (enzyme-linked immunosorbent assay) kit. This method is also complicated and needs to collect embryonic blood or allantoic fluid, and the accuracy rate reaches more than 98%. In addition, using spectral image technology to identify the differences in material composition and physical properties in eggs, and using machine learning to establish a spectral model for identifying male and female embryos, which is also a method for in ovo sexing. The method is non-invasive with high speed, but it has limited accuracy and poor generalizability. In terms of industrial applications, the SELEGGT company (Germany) has developed a rapid test reagent for sex hormones and launched commercial chicken embryo gender identification technology and equipment, which has been industrialized and applied with an accuracy rate of 98.5%. However, the identification speed is only 3 600 embryos/h, which is far lower than market expectations. Based on hyperspectral image technology, the AAT company (Germany) also launched a fully automatic chicken embryo gender identification equipment (CHEGGY). The identification speed reaches 20 000 embryos/h with an accuracy rate of 96%. However, it is only applicable to feather color self-differentiation layer breeds. From the perspective of the application requirements of the poultry industry in future, the gender identification technology based on the spectral image detection method has obvious advantages and the greatest potential for industrial application. It is necessary to clarify the physiological differences between male and female chicken embryos in the poultry embryonic development period and the physiological signal detection method, which will provide theoretical support for the development of high-efficiency in ovo sexing technology and equipment.

Key words: poultry, incubation, sex identification, spectral image, hormones, genes