Abstract
Comprehensive dissections of three infants with trisomy‐13 and three with trisomy‐18 have revealed several muscles which appear to be “abnormal” at the time of birth and shortly thereafter. Careful observations show that the peculiar morphology of these muscles results from delayed development rather than from anatomical malformation. The observations are compared with physiological, pathological and in vitro experimental studies of delayed embryonic development in human aneuploidy.