## Core Concept
Gastrulation is a critical phase in early embryonic development, occurring after the blastula stage. It involves the complex process of cell migration, differentiation, and organization, leading to the formation of the three primary germ layers: ectoderm, endoderm, and mesoderm. These layers eventually give rise to all tissues and organs in the body.

## Why the Correct Answer is Right
The correct answer, , states that gastrulation involves the formation of the three primary germ layers. This process is essential for the development of the embryo and is characterized by the movement of cells from the outer layer of the blastula (blastoderm) to the interior, forming the gastrula. During gastrulation, the **blastopore** forms, which is an opening that leads to the archenteron (primitive gut). The three primary germ layers formed during gastrulation are:
- **Ectoderm**: gives rise to the central nervous system, skin, and several outer body parts.
- **Endoderm**: develops into the inner lining of the digestive tract, liver, pancreas, lungs, and other internal organs.
- **Mesoderm**: forms the heart, muscles, bones, blood vessels, and connective tissues.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because gastrulation does not simply involve the formation of a single-layered blastula; rather, it leads to the development of a multi-layered embryo with distinct germ layers.
- **Option B:** This option is incorrect as it inaccurately describes the outcome of gastrulation. Gastrulation does not result in the formation of a hollow ball of cells (which describes the blastula) but in the formation of a multi-layered structure.
- **Option C:** Without the specific details of this option, we acknowledge that any statement contradicting the established understanding of gastrulation as the process leading to the formation of the three primary germ layers would be incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that defects during gastrulation can lead to severe congenital anomalies. For example, failure of proper **neurulation** (a process closely related to gastrulation) can result in neural tube defects such as spina bifida or anencephaly. Understanding the timing and processes of embryonic development is crucial for recognizing potential developmental abnormalities.

## Correct Answer Line
**Correct Answer: D. Gastrulation involves the formation of the three primary germ layers.**