FIVE FIGURES
A new mutation of fowl resulting in a rumpless condition has recently been discovered and investigated by Landauer ( '41, '45). Quite unlike the dominant rumpless mutation which had been described earlier*by Dunn ( '25) and Landauer ( '28)' this new variant behaves as a recessive on a Leghorn background. I n general the chicks from this recessive strain resemble those from the dominant stock, but several significant differences exist (Landauer, '45). The remaining caudal vertebrae tend to be irregular and incomplete, and often are deflected ventrally; scolioses, kyphoses, and lordoses occur frequently ; supernumerary ribs are encountered in a significantly greater number of cases than in normal controls. As in the dominant strain all degrees in the reduction of the tail are found, probabβ¬y as the result of the action of genetic modifiers.
A fairly large proportion of these chicks can not be distinguished from the normal by morphological criteria although genetic evidence indicates that they are homozygous for the rumpless factor. These have been interpreted as extreme manifestations of the action of modifiers.
The present report is the result of a study of the embryogeny of these recessive rumpless chicks. This work was undertaken to compare the mode of origin of the recessive rumpless condition with that previously determined for dominant rumplessness (Zwilling, '42) and to find whether the pheno-typically normal chicks bearing this mutation deviate from the normal during development.
MATERIAL AND METHODS
Pedigreed eggs were obtained from three matings of rumpless chickens. These matings had Been made up for selection experiments and the birds in each had been chosen because of the degree of rumplessness which they manifested. Thus pen 12 contained completely or nearly completely rumpless hens, pen 17 was made up of intermediates, and pen 18 of nearly normals. All classes of embryos were obtained from 79