Which of the following options describes the correct outer-to-inner sequence of tissue layers in a post-gastrulation vertebrate embryo?

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BlackTom AI · Accepted Answer

The question asks for the correct outer-to-inner sequence of tissue layers formed after gastrulation in a vertebrate embryo. 
Option 1: ectoderm → mesoderm → endoderm. This matches the standard organization where the ectoderm is the outermost germ layer, the mesoderm lies between the two, and the endoderm is the innermost layer lining the gut tube. 
Option 2: ectoderm → endoderm → mesoderm. This places the endoderm between ectoderm and mesoderm, which does not reflect the typical inner-to-outer order established during gastrulation. 
Option 3: mesoderm → endoderm → ectoderm. Listing mesoderm as the outermost layer contradicts the established topology where ectoderm is outermost; thus this is inconsistent with the developmental sequence. 
Option 4: endoderm → ectoderm → mesoderm. This arrangement starts with endoderm as outer, then ectoderm, then mesoderm interior, which is not how the germ layers are organized in the post-gastrulation embryo. 
In summary, evaluating each option against the well-supported pattern—ectoderm on the outside, mesoderm in the middle, endoderm on the inside—the first option aligns with the known biology, while the others misorder the layers and conflict with established embryology concepts.

Bio 93 Section CDE 2025 Final Exam Practice, Part 1

Which of the following options describes the correct outer-to-inner sequence of tissue layers in a post-gastrulation vertebrate embryo?

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