In animals, an animal zygote formed after fertilisation undergoes repeated cell divisions to develop into which of the following early embryonic stages?

Introduction / Context:
Developmental biology deals with how a single fertilised egg cell, called a zygote, develops into a complex multicellular organism. One of the first topics taught is the sequence of early embryonic stages, such as zygote, morula, blastula, and gastrula. This question asks what structure is formed after the zygote undergoes repeated cell divisions in animals, testing your understanding of basic embryology terminology.

Given Data / Assumptions:

• The starting point is an animal zygote formed by the fusion of male and female gametes.
• We are considering the normal embryonic development sequence.
• The options are egg, sperm, flagella, and blastula.
• We assume the standard order: zygote, cleavage, morula, blastula, and then further stages.

Concept / Approach:
After fertilisation, the zygote does not immediately grow in size but undergoes rapid mitotic divisions called cleavage. These divisions produce a solid ball of cells known as the morula, which then rearranges to form a hollow ball of cells enclosing a fluid filled cavity. This hollow stage is called the blastula. The blastula is therefore the structure that results from the repeated division and reorganisation of the zygote. Egg and sperm are gametes that exist before fertilisation, and flagella are whip like locomotory structures, not developmental stages. Hence, blastula is the correct answer.

Step-by-Step Solution:
Step 1: Start with the zygote, which is the single cell formed immediately after fertilisation. Step 2: Recall that the zygote undergoes multiple mitotic divisions called cleavage without significant increase in overall size. Step 3: Recognise that these divisions first produce a compact mass called the morula and then a hollow, single layered structure called the blastula. Step 4: Note that the question asks for the early embryonic stage formed after such repeated divisions, which corresponds to the blastula stage. Step 5: Compare the options and select blastula, rejecting those that are gametes or unrelated structures.

Verification / Alternative check:
To verify, think of standard diagrams in embryology: you usually see a sequence labelled zygote, 2 cell, 4 cell, 8 cell, morula, blastula, and then gastrula. The term blastula is always associated with a hollow sphere of cells early in development. Since the question explicitly mentions the zygote dividing to form something, the next named stage you normally study is blastula. This commonly taught order of events confirms that blastula must be the correct option.

Why Other Options Are Wrong:
An egg is a gamete present before fertilisation, not a structure formed after the zygote divides. Sperm is the male gamete that fuses with the egg to form the zygote; it is not formed from the zygote. Flagella are locomotory organelles present on some cells, not an embryonic stage. None of these options represents the early hollow ball of cells formed after cleavage divisions. Therefore, they cannot be correct answers to this question.

Common Pitfalls:
Some students may confuse morula and blastula or may forget the exact sequence of stages. Another pitfall is to misread the question and think it is asking what combines to form a zygote rather than what the zygote develops into. Carefully noting that the zygote divides to form the next structure prevents this confusion. Remembering the correct order as zygote to morula to blastula helps you quickly identify blastula as the early embryonic stage formed after repeated zygote divisions.

Final Answer:
The correct choice is Blastula, the early hollow ball of cells that develops from an animal zygote after repeated mitotic divisions.