Question

In Amphibia, gastrulation begins at:

• A. The part of the egg opposite to the entry of the sperm.
• B. Animal pole
• C. Vegetal pole
• D. Dorsal blastopore lip.

Hint:

The dorsal blastopore lip is the "director" of gastrulation. It's not just a location, but an active organizer that initiates and controls the complex cell movements that shape the embryo.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Gastrulation is the process of forming the three primary germ layers. In amphibians, this process is initiated at a specific location on the surface of the blastula.

Step 2: Detailed Explanation:
- The initiation point of gastrulation in an amphibian embryo is marked by the formation of a slit-like invagination called the blastopore.
- The first part of this structure to form is the dorsal blastopore lip. This is the precise site where the first cells begin to involute (roll inward) to form the mesoderm and endoderm.
- The location of the dorsal blastopore lip is determined earlier in development. Sperm entry triggers a cortical rotation, which establishes the future dorsal-ventral axis. The dorsal blastopore lip forms in a region known as the gray crescent, which appears on the side of the egg opposite to the point of sperm entry.
- While option (A) is correct in describing where the process is located relative to sperm entry, option (D) names the specific anatomical structure where the cellular movements of gastrulation physically begin. Therefore, (D) is the more precise and direct answer. The animal and vegetal poles (B and C) are the top and bottom ends of the embryo, respectively, not the starting point of gastrulation.

Step 3: Final Answer:
Gastrulation begins at the specific structure known as the dorsal blastopore lip. This structure is also famously known as the "Spemann-Mangold organizer" because it orchestrates the development of the entire embryonic axis. Option (D) is the correct answer.