Early Embryology of the Cow. I. Gastrula and Primitive Streak Stages

Serial sections of 16 bovine embryos recovered from cows slaughtered at 16, 17, and 18 days after insemination were studied to establish suitable standards of normal development and to define the limits of normal variation encountered during early pregnancy. The 16-day-old blastocysts were engaged in the processes of gastrulation, and ranged in development from a hollow bilaminar sphere to an elongate chorionic vesicle with an elevated germinal disk. Their principal features were the remarkably rapid growth of the trophoblast and the differentiation of somatic and splanchnie mesoderm. Blastoeysts recovered at 17 days were characterized by further growth, cephalocaudal elongation of the germinal disk, and differentiations of Hcnsen's node and the primitive streak establishing the longitudinal axis. In the 18th day of development, the blastocysts extended throughout the major portion of the gravid horn and the embryos displayed well-defined primitive node, primitive groove, and notochord development. Amniogenesis was initiated and, in two specimens, completed amniotic sacs were present. Two cases of probable embryonic death were encountered in which the trophoblast persisted after loss and resorption of the embryonic area. The morphogenetic processes involved in gastrulation and primitive streak formation are considered in the light of reported high embryonic death during early pregnancy.