gastrulation

Meaning: Gastrulation follows cleavage and blastula formation. Cells move inward in organized migrations, turning a hollow ball of cells into a gastrula with three primary germ layers: ectoderm (outer), mesoderm (middle), and endoderm (inner). In sea urchins and amphibians, cells invaginate at the blastopore and form the archenteron, the primitive gut. In birds and mammals, cells enter through a primitive streak instead. The blastopore becomes the anus in deuterostomes (including humans) and the mouth in protostomes.

On the MCAT: Expect questions that match each layer to its derivatives. Ectoderm gives the epidermis and nervous system. Mesoderm gives muscle, bone, the circulatory system, kidneys, and gonads. Endoderm gives the lining of the digestive and respiratory tracts and organs such as the liver and pancreas. Passages may also ask you to order the stages (zygote, cleavage, morula, blastula, gastrula, neurulation, organogenesis) or to predict which tissues fail to form if one layer is disrupted.

Common confusion: Gastrulation is not just more cleavage. Cleavage divides the zygote into smaller cells without an increase in overall size, while gastrulation rearranges cells to create the layers. It also comes before neurulation, the later step in which ectoderm folds into the neural tube.