**Core Concept**
The development of an embryo is a complex process involving multiple cell divisions, differentiation, and morphogenesis. It begins with fertilization, where a sperm fuses with an egg to form a zygote, which then undergoes several cleavage stages to form a blastocyst.

**Why the Correct Answer is Right**
The correct answer is related to the concept of implantation, where the blastocyst attaches to the uterine lining, initiating embryonic development. This process involves the trophoblast cells of the blastocyst invading the uterine epithelium, leading to the formation of the placenta and the establishment of a nutrient and waste exchange system. The embryoblast cells of the blastocyst then undergo gastrulation, giving rise to the three primary germ layers: ectoderm, endoderm, and mesoderm.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the process of embryonic development.
**Option B:** This option is incorrect because it is not relevant to the development of an embryo.
**Option C:** This option is incorrect because it refers to a different biological process.

**Clinical Pearl / High-Yield Fact**
The process of implantation is critical for successful embryonic development and is influenced by various factors, including hormonal changes, uterine receptivity, and the presence of a competent embryo.

**Correct Answer: A. The trophoblast cells of the blastocyst invade the uterine epithelium.**