The embryonal period in sheep is the period from the end of blastogenesis to day 34. Sheep embryos, 3-4 days after fertilization enters the uterus. The zygote formed by the union of male and female pronuclei undergoes mitotic divisions. First, it becomes an embryo with two blastomeres. On the 14th day after mating, the filamentous conceptus is immobilized in the uterine lumen. The elongated blastocyst maintains close contact with the endometrial epithelium. In all mammals, the endometrial uterine glands secrete a complex fluid containing proteins called histotrophs and various nutrients. In this secretion product; enzymes, growth factors, cytokines, lymphokines, hormones, transport proteins etc. is found. In histotrophic nutrition, proteins and other nutrient molecules produced by the uterine glands ensure the survival of the embryo. The embryo floats in this histotrophic fluid. In non-pregnant sheep, 16-17 days of the cycle. The release of Prostaglandin (PGF₂α)  from the endometrium increases and causes regression of the corpus luteum. Therefore, inhibition of the secretion of endometrial PGF2α is essential in the acceptance of pregnancy. In sheep, the free-floating blastocyst produces specific proteins in the uterus. This protein is called ovine trophoblastic protein-1 (oTP-1) and is known as interferon tau. Interferon tau produced in the trophectoderm of the blastocyst is explained as inhibiting the synthesis of oxytocin receptors by suppressing endometrial estradiol receptors and thus inhibiting the secretion of PGF2α. Interferon tau, 10-21 days of pregnancy in sheep. It is produced on the 15th day and reaches its maximum level on the 15th day. The circulating concentration of progesterone hormone ensures survival and development of the blastocyst during early pregnancy.