Question

Arrange the following events in correct sequence:

• [A.] Formation of zygote
• [B.] Formation of blastocyst
• [C.] Implantation
• [D.] Formation of morula

• A. A, D, B, C
• B. A, B, D, C
• C. A, B, C, D
• D. C, D, B, A

Hint:

Remember the embryonic developmental order: \[ \text{Zygote} \rightarrow \text{Morula} \rightarrow \text{Blastocyst} \rightarrow \text{Implantation} \] Morula is solid, while blastocyst contains a fluid-filled cavity called blastocoel.

Answer 1

The Correct Option is A

Concept: Human embryonic development begins immediately after fertilization. The fertilized egg undergoes a series of mitotic divisions known as cleavage while moving through the fallopian tube toward the uterus. During this journey, the developing embryo passes through several important stages before implantation occurs inside the uterine wall. The major stages involved are: \[ \text{Zygote} \rightarrow \text{Morula} \rightarrow \text{Blastocyst} \rightarrow \text{Implantation} \] Each stage is structurally and functionally important for successful establishment of pregnancy.

Step 1: Formation of zygote.
Fertilization occurs when the male gamete (sperm) fuses with the female gamete (ovum). As a result of this fusion:

• Male and female pronuclei unite
• Diploid chromosome number is restored
• A single-celled structure called zygote is formed

The zygote is therefore the first cell of the new individual. Hence, event (A) occurs first.

Step 2: Formation of morula.
After formation of zygote, rapid mitotic divisions called cleavage begin. During cleavage:

• Cell number increases rapidly
• Overall size of embryo remains nearly same
• Smaller cells called blastomeres are formed

When the embryo reaches approximately 8--16 blastomeres, it forms a solid ball-like structure known as morula. The term morula is derived from the Latin word meaning “mulberry,” because of its appearance. Thus, event (D) occurs after zygote formation.

Step 3: Formation of blastocyst.
As cleavage continues further, the morula develops a fluid-filled cavity called blastocoel. At this stage, the embryo becomes a blastocyst. The blastocyst consists of:

• Outer trophoblast cells
• Inner cell mass

The trophoblast later contributes to placenta formation, while the inner cell mass develops into the embryo proper. Hence, event (B) occurs after morula formation.

Step 4: Implantation of blastocyst.
The blastocyst reaches the uterus and attaches itself to the endometrial lining. The trophoblast cells invade the uterine wall and embed the blastocyst inside the endometrium. This process is known as implantation. Implantation is extremely important because:

• It establishes pregnancy
• It allows nourishment from mother
• It initiates placental development

Thus, implantation occurs after blastocyst formation. Hence, event (C) occurs last.

Step 5: Determine the final correct sequence.
Combining all stages in proper order: \[ \text{Zygote} \rightarrow \text{Morula} \rightarrow \text{Blastocyst} \rightarrow \text{Implantation} \] Therefore, the correct sequence is: \[ A \rightarrow D \rightarrow B \rightarrow C \] Hence, the correct answer is: \[ \boxed{(A)\ A, D, B, C} \]