Question

When all three germ layers are present (ectoderm, endoderm, and mesoderm), what event has occurred? a. blastulation b. limb formation c. gastrulation d. morulation

Short answer

The correct answer is c. gastrulation, as this is the stage in embryonic development when all three germ layers (ectoderm, endoderm, and mesoderm) are formed and the basic embryonic body plan is established.

Step-by-step solution

Understand the early stages of embryonic development

To solve this problem, it's important that you understand the different stages of embryonic development, which include morulation, blastulation, and gastrulation. Each of these stages serves a specific purpose and results in specific structures. This will help you recognize which event aligns with the presence of all three germ layers.

Morulation

Morulation is the first stage of embryonic development in which the fertilized egg, or zygote, undergoes a series of cell divisions (cleavage) to form a solid ball of embryonic cells called a morula. This stage does not involve the formation of germ layers.

Blastulation

Blastulation is the next stage of embryonic development where the morula turns into a hollow ball of cells called the blastula, which contains an inner cavity called the blastocoel. At this stage, there is still only one germ layer present - the ectoderm.

Gastrulation

Gastrulation is the critical stage where all three germ layers (ectoderm, endoderm, and mesoderm) are formed, and the basic embryonic body plan is established. This is achieved through a series of complex cell movements and rearrangements, which result in the creation of the primitive streak and eventually the gastrula.

Limb formation

Although limb formation is an important event in embryonic development, it occurs after the formation of the three germ layers and is not associated with their initial presence.

Identify the correct answer

Based on the understanding of each event in early embryonic development, it is clear that gastrulation is the event that occurs when all three germ layers (ectoderm, endoderm, and mesoderm) are present. Therefore, the correct answer to this question is: c. gastrulation