Biological Systems MCAT 2025 – 400 Free Practice Questions to Pass the Exam

During gastrulation, the developmental process that occurs after the blastula stage, the embryo transforms from a single-layered structure into a multi-layered entity known as the gastrula. Specifically, gastrulation results in the formation of three distinct germ layers: the ectoderm, mesoderm, and endoderm. Each of these layers is crucial because they give rise to different tissues and organs in the developing organism.

The ectoderm will eventually form structures such as the skin and nervous system, the mesoderm contributes to muscles, bones, and the circulatory system, while the endoderm is responsible for forming the lining of the digestive tract and associated organs. This layering is a fundamental aspect of development as it lays the groundwork for the intricate organization and specialization of cells that follows in later stages of embryogenesis.

Other choices represent different stages or structures irrelevant to the gastrulation process: a single-layered blastula refers to the embryonic stage prior to gastrulation, the placenta is a structure formed in later stages of pregnancy for nutrient and waste exchange between mother and fetus, and the morula is an earlier stage of development consisting of a ball of cells formed right after fertilization but before the formation of the blastula.