Question

Arrange the following stages of development in a typical teleost, starting from the earliest
A. Gastrula
B. Cleavage
C. Blastula
D. Fertilization
E. Morula
Choose the correct answer from the options given below:

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

Hint:

Standard Embryology Sequence:
Fertilization $\rightarrow$ Cleavage $\rightarrow$ Morula $\rightarrow$ Blastula $\rightarrow$ Gastrula.
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Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the correct chronological sequence of embryonic developmental stages in teleost (bony) fishes.
Detailed Explanation:

• Fertilization (D): This is the very first step where the sperm penetrates the egg (ovum), leading to the formation of a zygote.

• Cleavage (B): Following fertilization, the zygote undergoes rapid mitotic cell divisions without significant growth. In teleosts, this is typically discoidal meroblastic cleavage at the blastodisc.

• Morula (E): A solid ball of cells (blastomeres) is formed as a result of initial cleavage. This stage is called the morula because of its resemblance to a mulberry.

• Blastula (C): As divisions continue, a fluid-filled cavity called the blastocoel forms within the cell mass. In fish, the cells sit on top of the yolk as a blastoderm.

• Gastrula (A): This stage involves morphogenetic movements (like epiboly, involution, and ingression) where the single-layered blastula is transformed into a multi-layered structure with primary germ layers (ectoderm, mesoderm, endoderm).

Step 2: Final Answer:
The correct sequence is D (Fertilization) $\rightarrow$ B (Cleavage) $\rightarrow$ E (Morula) $\rightarrow$ C (Blastula) $\rightarrow$ A (Gastrula). This corresponds to Option (D).