What is the correct observation regarding size of egg, blastula and gastrula?
(a) There is progressive increase in size from zygote to blastula to gastrula
(b) All the three are of same size
(c) Zygote is small while blastula and gastrula are larger
(d) Gastrula is larger while zygote and blastula are of same size

Options

A. There is progressive increase in size from zygote to blastula to gastrula
B. All the three are of same size
C. Zygote is small while blastula and gastrula are larger
D. Gastrula is larger while zygote and blastula are of same size.

Answer

To solve the question regarding the size of the egg, blastula, and gastrula, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Zygote**: The zygote is the initial cell formed when a sperm fertilizes an egg. It is relatively small and is surrounded by a protective layer called the zona pellucida.

**Hint**: Remember that the zygote is the first stage of development after fertilization.

2. **Cleavage Process**: After the formation of the zygote, it undergoes cleavage, which is a series of rapid cell divisions. During this process, the zygote divides into smaller cells, but the overall size remains the same. The zygote and the resulting morula (a solid ball of cells) are of the same size.

**Hint**: Cleavage involves cell division without growth, keeping the size constant.

3. **Formation of Blastula**: The morula then transforms into a blastula through further cleavage. At this stage, the cells are still dividing, and the size of the blastula remains similar to that of the zygote and morula.

**Hint**: The blastula is formed from the morula but does not increase in size during this stage.

4. **Gastrulation**: After the blastula stage, the process of gastrulation occurs. During gastrulation, the cells begin to rearrange and differentiate, leading to the formation of the gastrula. Importantly, the zona pellucida degrades during this process, allowing the gastrula to expand.

**Hint**: Gastrulation is a key stage where the embryo begins to form distinct layers and increases in size.

5. **Comparison of Sizes**: By the end of these stages, we can conclude that:
- The zygote and blastula are of the same size.
- The gastrula is larger than both the zygote and the blastula.

**Hint**: Focus on the fact that the gastrula is the only stage that shows a significant increase in size compared to the zygote and blastula.

### Conclusion:
Based on the observations, the correct answer to the question is:
**(d) Gastrula is larger while zygote and blastula are of the same size.**