are gestational sac and yolk sac the same
Gestational sac The yolk sac and embryo should be readily identifiable when the gestational sac reaches a certain size â€” a yolk sac should be seen when the gestational sac is 20mm and a fetal pole should be seen when the gestational sac reaches 25mm.
Embryo In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of blastula, gastrula, and organogenesis. The blastula stage typically features a fluid-filled cavity, the blastocoel, surrounded by a sphere or sheet of cells, also called blastomeres. In a placental mammal, an ovum is fertilized in a fallopian tube through which it travels into the uterus. An embryo is called a fetus at a more advanced stage of development and up until birth or hatching. In humans, this is from the eleventh week of gestation. However, animals which develop in eggs outside the mother's body, are usually referred to as embryos throughout development; e.g. one would refer to a chick embryo, not a "chick fetus", even at later stages.