Postnatal somatometry of the rat facial skeleton.

Growth of the rat facial skeleton over a 40 day period from birth was examined relative to 8 length and 4 width parameters of animals subject to somatic growth retardation experimentally induced by overtaxing the maternal lactational capacity by means of excessively large cross-fostered litters. Least squares regression lines were calculated for the preweaning period (day 1-20); the postweaning period (day 21-40) and the total period (day 1-40), thereafter being recalculated into segmented regression lines relative to the physiological growth phases. Specific phases of growth activity were noted throughout the 40 days with those in the preweaning period showing a marked consistency concomitant with the maternal lactational ability and subsequent emergence of biological weaning. A consistent phasic growth spectrum with definitive breaks in continuity supports the original postulate (Brody, 1927) that growth curves consist of phases of exponential activity delineated by changes in growth rate. Due to the inclusion of both control and experimental samples, the breakage points of the phases marking the change of growth rate different and were therefore reciprocated by alterations of phasic duration. Phasic growth occurred in the postweaning period with some degree of variance which did not permit an assessment of the causation factors with sufficient validity. The postweaning period of the experimental sample exhibited varying degrees of growth recovery (catch-up) in the majority of the facial parameters. Covariance analysis of the parameters in most instances showed that the parameters, as defined, and the bone units incorporated within them manifested a characteristic growth response in each sample irrespective of the modifying influence of the environmental factors.