Abstract
This paper is dedicated to Professor L. C. Dunn ( Columbia University, New York ) in recognition of his long and distinguished career . Mice heterozygous for the T s gene have shorter tails and smaller bodies than their normal litter-mates. A study of their anatomy revealed skeletal abnormalities of various kinds scattered over the whole body. These abnormalities can be traced back to 11-day embryos, and are always accompanied by anaemia (which disappears before birth) and the consequent retardation. The anaemia in turn can be traced back still further to a deficiency of blood islands in the yolk sac of 8-day embryos. It is suggested that the primary effect of the gene is anaemia, which, acting through differential retardation of various organs, leads to the abnormalities seen later.