the fertilized egg, which is diploid, that results from the union of a sperm cell nucleus and an egg cell nucleus.

chemical messenger produced in one part of the body that controls the activity of other parts.

developing stage of multicellular organism; in humans, the stage in the development of offspring from the first division of the zygote until body structures begin to appear, about the ninth week of gestation.

nerve cell that characteristically has three parts: dendrites, cell body, and an axon.

maintenance of normal internal conditions in a cell or an organism by means of self-regulating mechanisms.

a junction between two neurons, or between a neuron and an effector cell; electrical or chemical signals are relayed from one cell to another at a synapse.