Ontogeny and Developmental Biology
Core concepts
Know ontogeny as individual development from zygote to adult. Understand cleavage, blastula, gastrula, and organogenesis. Know the germ layers: ectoderm, mesoderm, endoderm, and the organs each gives rise to. Understand embryonic induction, differentiation, and the role of the genome in development. Know direct vs indirect development (with larval stages and metamorphosis). In plants, know seed germination, primary growth from apical meristems, and secondary growth from lateral meristems. Understand the biogenetic law's modern interpretation and the concept of recapitulation.
Traps
Differentiation does not change the genome; it changes gene expression. Cleavage increases cell number without increasing mass. The blastula precedes the gastrula. Ectoderm gives skin and nervous system; mesoderm gives muscle, bone, blood; endoderm gives gut lining and associated organs. Metamorphosis is indirect development. Do not confuse meristematic (dividing) tissue with permanent tissue.
Worked example / rule of thumb
Rule of thumb: "three layers, three fates: outside, middle, inside." Worked example: the nervous system derives from ectoderm, the skeleton and muscles from mesoderm, and the lining of the digestive tract from endoderm. If a question asks why a liver cell differs from a neuron despite identical DNA, the answer is differential gene expression during development.