It has long been known that meristic characters, and especially the number of vertebrae, in many fishes are affected by environmental factors such as temperature, salinity, oxygen concentration, and light during early development. The temperature response curve of vertebral number for offspring from a single mating is often Ushaped, but may be inversely linear or some other shape. Less is known about response to other variables, but there is generally a clear trend within the tolerance range. The genotype of a fish significantly affects the response curve of vetebral number to environmental factors. A few studies have shown that tetrapod meristic characters are also sesitive to temperature and light. Earlier reviews and recent experiments papers are cited d analyzed to show that these phenomena can now be investigated experimentally. Metabolic rate has been correlated with vertebral number and the possible implications of this are discussed. Theories accounting for the formation of somites are examined. The customary methods of measuring development are regarded as inexact and unsuitable for explanation of the phenomena discussed here. Two interesting evolutionary implications are discussed: teh absence of genetic assimilation of vertebral number, and the apparent absence of selection for a single specific vertebral number in each population.