Postnatal skull growth of the rat relative to environmental variance.

Rat growth during the first 40 days of postnatal development was examined relative to cranial length, facial height and bizygomatic width of animals subject to nutritional suppression induced by increased litter size during the preweaning period. The postulate (Brody, 1927) that growth curves consist of phases of exponential activity delineated by alterations in the growth rate thus inducing breaks in the continuity was confirmed in the skull. The preweaning and postweaning periods showed three clearly demarcated phases of growth within each period. Breakage points of the phases--alteration in growth rate--differed between the control and experimental litters thus changing the length of the specific phases. Within the preweaning period of both types of litter the phasic characteristics appeared directly linked to the maternal capacity--quantitatively and in duration, while the latter part of the preweaning period and the postweaning period remained within the province of external or non-maternal food supplies. The postweaning of the experimental animals exhibited catch-up growth phenomena in all parameters with a slight variation relative to catch-up growth phenomena in all parameters with a slight variation relative to the sexes. In general terms, analysis of the total period of investigation indicated that growth within the preweaning period was more active although sexual dimorphism was not significant. In addition, relative growth (allometric) analysis of the three parameters showed varying growth rates, breakage points and hence phasic lengths indicating that irrespective of environmental factors that each parameter and the bone units incorporated in it manifested a response as a whole which was characteristic of the parameter.