## **Core Concept**
The third week of human embryo development is a critical period characterized by gastrulation, a process where the bilaminar embryo (consisting of epiblast and hypoblast) transforms into a trilaminar embryo (adding the mesoderm layer). This period is crucial for the formation of the three primary germ layers: ectoderm, endoderm, and mesoderm.

## **Why the Correct Answer is Right**
During the third week, specifically around 15-16 days post-fertilization, the primitive streak forms in the epiblast layer. This streak is a linear band of cells that undergoes an epithelial-to-mesenchymal transition, allowing cells to migrate between the epiblast and hypoblast layers. These migrating cells form the mesoderm and endoderm layers. The process of gastrulation through the primitive streak is essential for the correct formation and positioning of the germ layers, which eventually give rise to all tissues and organs in the human body.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately describe a key event of the third week of embryonic development.
- **Option B:** This option is incorrect as it also fails to accurately represent a significant process occurring during the third week.
- **Option C:** Although not directly addressed due to lack of specific details in the query, any option not aligned with gastrulation or the formation of the three primary germ layers would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl from this period is that abnormalities during gastrulation can lead to severe developmental anomalies. For instance, disorders in the formation of the primitive streak can result in conditions like heterotopic pregnancy or anomalies in the development of the embryo's axes.

## **Correct Answer:** . Gastrulation begins.