Several years ago me observed in our mouse colony some newborn mice with peculiar abnormalities of head and extremities. The first abnormals appeared in a tailless strain (tailless line 11) and were the offspring of a tailless male and two of his tailless sisters. Two of the abnormals were tailless and one was a normal-tailed individual. Thc most striking deformities of these newborns were the extreme shortening of the extremities and the reduction in liead size. The abnormality mas termed ' ' phocomclia" because of the seal-like appearance of the young, althougli tlic actual morphology of the abnormal extremities would be described more accurately as micromelia. Breeding experiments, to be described below, soon showed that this abnormality was inherited as a simple reccssivc Mendelian character, and further study of the abnormal morphology revealed many abnormalities in addition to those of the extremities, combined in a syndrome incompatible with viabilit,y. Not a single abnormal young ever survived the first 24 hours after birth.

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