early embryogenesis

Meaning: Early embryogenesis is the sequence from fertilization to formation of the three germ layers. The zygote undergoes cleavage: rapid mitotic divisions with little growth, so cells (blastomeres) get smaller. The solid ball is the morula, which becomes a blastocyst with an outer trophoblast and an inner cell mass, and implants in the uterus. Gastrulation forms ectoderm, mesoderm and endoderm; neurulation follows, as the notochord induces ectoderm to form the neural tube.

On the MCAT: Expect questions that order the stages or match germ layers to tissues: ectoderm to epidermis and nervous system, mesoderm to muscle, bone, blood and kidneys, endoderm to the gut lining, liver and pancreas. Passages may also test potency (early blastomeres are totipotent; inner cell mass cells are pluripotent) or induction, where one tissue signals another to differentiate. In deuterostomes such as humans the blastopore becomes the anus; in protostomes it becomes the mouth.

Common confusion: Cleavage is not growth. The embryo's overall size stays about the same, so cell number rises while cell size falls. Also, the germ layers form during gastrulation, not cleavage.