Variation of the lumbar vertebrae of mice at two environmental temperatures.

The house mouse, Mus musculus L., usually has twenty-six presacral vertebrae: these consist of seven cervical, thirteen thoracic and six lumbar vertebrae (Weber, 1950; Deol, 1958; Berry & Searle, 1963; Berry, 1964). The twenty-sixth vertebra is, however, sometimes abnormal, especially among laboratory mice: on one or both sides it may be fused with the sacrum. There are then five typical lumbar vertebrae, instead of six. The proportion of such abnormalities in a population is influenced by genotype, diet and unidentified maternal factors (reviewed by Griineberg, 1963). Evidence is given below that a low environmental temperature can also increase the incidence of ' sacralization of L6'.