| A | B |
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| Apical meristems | embryonic plant tissue in the tips of roots and in the buds of shoots that supplies cells for the plant to grow in length. |
| Model organism | an organism chosen to study broad biological principles |
| Cell lineage | the ancestry of a cell |
| Totipotent | describing a cell that can give rise to all parts of an organism |
| Differentiation | cell differentiation |
| Morphogenesis | the development of body shape and organization |
| Determination | the progressive restriction of developmental potential, causing the possible fate of each cell to become more limited as the embryo develops. |
| Cytoplasmic determinants | The maternal substances in the egg that influence the course of early development by regulating the expression of genes that affect the developmental fate of cells. |
| Pattern formation | The ordering of cells into specific three dimensional structures, an essential part of shaping an organism and its individual parts during development. |
| Induction | The ability of one group of embryonic cells to influence the development of another. |
| Positional information | Signals to which genes regulating development respond, indicating a cell's location relative to other cells in an embryonic structure. |
| Embryonic lethals | A mutation with a phenotype leading to death at the embryo or larval stage. |
| Maternal effect genes | A gene that, when mutant in the mother, results in a mutant phenotype in the offspring, regardless of the genotype. |
| egg-polarity genes | Another name for a maternal effect gene, a gene that helps control the orientation (polarity) of the egg. |
| morphogens | A substance, such a Bicoid protein, that provides positional information in the form of a concentration gradient along an embryonic axis. |
| Segmentation genes | A gene of the embryo that directs the actual formation of segments after the embryo's axes are defined. |
| Homeotic genes | Any of the genes that control the overall body plan of animals and plants by controlling the developmental fate of group cells |
| homeobox | A 180-nucleotide sequence within homeotic genes and some other developmental genes that is widely conserved in animals. |
| Apoptosis | The changes that occur within a cell as it undergoes programmed cell death, which is brought about by signals that trigger the activation cascade of suicide proteins in the celll destined to die. |
| Chimeras | An organism with a mixture of genetically different cells. |
| Organ-identity genes | Plant homeotic genes that use positional information to determine which emerging leaves develop into which types of floral organs. |
