COMPARATIVE PATTERNS OF CRANIOFACIAL DEVELOPMENT IN EUTHERIAN AND METATHERIAN MAMMALS.

The sequence of differentiation of major elements of the skeletal, muscular and nervous systems of the head is examined in developmental series of five eutherian (placental) and four metatherian (marsupial) mammals. The analysis identifies the elements that are conserved across the Theria, those that are unique to the Metatheria and to the Eutheria, and those that are variable. It is shown that although there are slight shifts in the sequence of development within the somatic tissues of the head, the primary difference between marsupial and placental mammals involves the timing and rate of differentiation of structures of the central nervous system (CNS) relative to a specific subset of structures of the cranial skeleton and musculature. In eutherians, CNS morphogenesis is well underway before the somatic tissues of the head begin differentiation. In metatherians, CNS development is delayed considerably and certain elements of the skeletal and muscular systems are advanced. It is concluded that the developmental differences between marsupial and placental mammals are best explained by the interaction of several processes including neurogenesis as a potential rate-limiting step, the developmental requirements of somatic elements, and the extremely short period of organogenesis of marsupial mammals. Several other issues, including the way that these data may be applied to determine the primitive therian developmental condition, and the use of comparative developmental data to address basic questions on morphogenetic processes, are discussed.