During embryonic development, which stage is a hollow ball-like structure that typically attaches to the side of the uterus?

During embryonic development, which stage is a hollow ball-like structure that typically attaches to the side of the uterus?

• A. Blastula
• B. Zygote
• C. Morula
• D. Gastrula

Answer: (A)

The main idea is the hollow ball stage that can implant into the uterus. After fertilization, the embryo cycles through a solid ball of cells called a morula, then forms a fluid-filled cavity to become a hollow sphere known as a blastula (in mammals, the blastocyst). This hollow blastula is what typically attaches to the uterine lining during implantation—the outer cell layer (trophoblast) helps secure and connect with the endometrium to establish pregnancy. The zygote is a single cell, the morula is solid, and the gastrula comes after implantation when germ layers form, so they don’t describe the hollow, implanting stage.

The main idea is the hollow ball stage that can implant into the uterus. After fertilization, the embryo cycles through a solid ball of cells called a morula, then forms a fluid-filled cavity to become a hollow sphere known as a blastula (in mammals, the blastocyst). This hollow blastula is what typically attaches to the uterine lining during implantation—the outer cell layer (trophoblast) helps secure and connect with the endometrium to establish pregnancy. The zygote is a single cell, the morula is solid, and the gastrula comes after implantation when germ layers form, so they don’t describe the hollow, implanting stage.