The gestational sac, also known as the gestational sac, is the shape of the first embryo during pregnancy. The embryo is implanted into the endometrium in the second week, and when it develops to the second germ layer, a large cavity called the primary yolk sac appears on the ventral side of the blastoderm. The yolk sac is constantly shrinking and degenerating during embryonic development, but the ectoderm on the yolk sac wall is the earliest birthplace of blood cells, blood vessels and primordial germ cells-fetal sac, where the fetus will grow in the future.
Embryo refers to sexual reproduction, which refers to the larvae with the ability to develop into biological adults after the combination of male germ cells and female germ cells into zygotes and repeated cell division and cell differentiation. Embryo refers to the initial stage of sexual reproduction development, which is the earliest stage of developmental biology from the first division of fertilized eggs to the next development stage.