Possible "atavistic" structures in human aneuploids.

Comprehensive dissections of aneuploid (trisomy 18 and 13) neonates have revealed numerous supernumerary muscles that, although present in rare individuals, do not regularly occur in the human. These supernumerary muscles include: "platysma occipitalis," "rhomboideus occipitalis, "deltopectoral" complex, "latissimocondyloideus," "pectorodorsalis," "chondrohumeralis," and "peroneus digiti quinti"; a few muscles, e.g., palmaris longus and brevis are lacking altogether. One specimen exhibited a "linguofacial trunk" arising from the external carotid artery. The supernumerary muscles found in these aneuploid specimens are regularly found in monkeys and sometimes in the great apes. It is suggested that these supernumerary muscles may be "atavistic" structures. Problems in establishing homologies between these muscles among primates are discussed, and mechanisms leading to the development of these muscles in human aneuploids are proposed.