The Prenatal Development of the Dog: Preimplantation Events1

Prenatal development of purebred beagle dogs was studied from the time of ovulation to the time of implantation. Ovulation occurred 1–2 days after the bitch’s first acceptance of coitus. Primary oocytes were shed from the ovary, and the first evidence of polar body formation was observed 3 days after breeding. The embryos entered the uterus as morulae of 16 cells or more, 8–12 days after breeding, and they became blastocysts shortly thereafter. During the free-floating blastocyst stage, which lasted about 1 week, the blastocysts grew to 2.6 mm in diameter and migrated through the uterus. Implantation, first marked by local endometrial edema at the definitive site of each embryo, began an average of 17-18 days after breeding. Shortly after the occurrence of local endometrial edema, differentiation of the primitive streak and three primary germ layers began in the embryo. The age of embryos was assessed on the basis of the time elapsed from breeding, the time elapsed from refusal to breed, and the time after the vaginal smear showed a change from predominantly cornified to predominantly noncornified epithelial cells. The calculated age of embryos at a given stage of development varied as much as 7 days when either postbreeding or postrefusal ages were used. The variation was reduced to 1–2 days when age was determined according to characteristics of the vaginal smear. The early embryonic development in the dog appears to be more closely associated with hormonal events near the end of estrus than with the time of ovulation. The vaginal smear is the best clinical index of these events.