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

Key concepts

Embryonic Development

Embryonic development is the remarkable process by which a single fertilized egg, or zygote, gradually transforms into a fully-formed organism. This journey begins immediately after fertilization and involves a series of well-coordinated stages. Each stage of development plays a crucial role in preparing the embryo for the next step in its journey. The key stages in embryonic development include:

• Morulation
• Blastulation
• Gastrulation

One fascinating aspect of this process is its precision and the sequential emergence of complexity, where simple cellular structures evolve into specialized tissues and organs.

Germ Layers

Germ layers are fundamental cellular layers that form during the early stages of embryonic development. These layers eventually give rise to all tissues and organs in the body. There are three primary germ layers:

• Ectoderm: This is the outermost layer and it gives rise to the skin, hair, nails, and the nervous system.
• Endoderm: The innermost layer that forms the lining of the gut, liver, pancreas, and lungs.
• Mesoderm: The middle layer, which will become the muscles, bones, heart, and circulatory system, among other structures.

Understanding these germ layers is vital because their formation marks a pivotal moment in development—setting the stage for the intricate process of organogenesis.

Morulation

Morulation is the initial stage in embryonic development after fertilization. During this phase, the zygote undergoes rapid cell division, known as cleavage, which results in a compact cluster of cells referred to as a morula. Despite the increase in cell number, the overall size of the embryo does not significantly change at this point. Key characteristics of morulation include:

• Formation of a solid ball of cells
• Absence of germ layer differentiation

Though vital for subsequent stages, morulation does not yet establish the diverse layers needed for developing specialized tissues and organs.

Blastulation

Following morulation, the embryo enters the blastulation stage. In this phase, the morula transforms into a structure called a blastula. The blastula is characterized by the formation of a hollow cavity known as the blastocoel. This stage signals further organization of cells. Important features of blastulation include:

• Formation of the blastocoel, which is a fluid-filled cavity
• Presence of only the ectoderm at this stage

Although the germ layers are not fully differentiated at this stage, blastulation is a preparatory step that sets the foundation for the upcoming critical phase—gastrulation.