**Core Concept**
The question is testing the understanding of the development of the immune system in a human embryo. The presence or absence of leukocytes (white blood cells) during different prenatal stages is crucial for understanding the development of the immune system. The development of the immune system is a complex process, and the presence of leukocytes is a key indicator of this process.

**Why the Correct Answer is Right**
Leukocytes are not present in the developing embryo during the first week after fertilization, which is known as the blastocyst stage. This is because the blastocyst stage is characterized by the absence of blood vessels and the formation of the embryoblast and trophoblast. The embryoblast differentiates into the three primary germ layers, and the trophoblast forms the placenta and other supporting tissues. Leukocytes are not formed until the development of the hematopoietic system, which occurs later in embryonic development.

**Why Each Wrong Option is Incorrect**
* **Option A:** The morula stage is a stage of embryonic development that occurs after the blastocyst stage, and leukocytes are not present in the morula.
* **Option B:** The gastrulation stage is a stage of embryonic development that occurs after the morula stage, and leukocytes are not present in the gastrula.
* **Option C:** The organogenesis stage is a stage of embryonic development that occurs after the gastrulation stage, and leukocytes are present in the developing embryo during this stage.

**Clinical Pearl / High-Yield Fact**
The development of the immune system is a complex process that occurs over several weeks, and the presence of leukocytes is a key indicator of this process. Understanding the timing of leukocyte development is crucial for diagnosing and treating immune-related disorders.

**Correct Answer:** D.