Human Reproduction

Early human development from fertilization to implantation is traced in NCERT Class 12, Chapter 3 (Human Reproduction). After syngamy in the ampullary region, the zygote undergoes cleavage as it travels down the fallopian tube, forming a morula, which then develops a fluid-filled cavity to become a blastocyst. It is the blastocyst, with its outer trophoblast and inner cell mass, that reaches the uterus and embeds itself in the endometrium during implantation. The morula is incorrect because at this compact, mulberry-like stage the embryo is still moving through the tube and has not yet acquired the trophoblast needed to attach. A two-celled zygote is far too early, representing only the first cleavage division. Gastrula is wrong because gastrulation, the formation of germ layers, occurs after implantation has already taken place. A developmental sequence check confirms the answer: morula precedes blastocyst, and only the blastocyst stage implants, matching the question exactly.