Prenatal size-age relationships and external morphology in the Mongolian gerbil (Meriones unguiculatus).

Abstract

AbstractEmbryos from timed matings were studied at days 12–24 of gestation with respect to crown‐rump length and external appearance. A linear increase in length was observed from the twelfth (2.5 mm) to the twenty‐fourth (27.7 mm) day with the largest increases occurring between days 20 and 21 (3.8 mm) and days 22 and 23 (4.2 mm). The smallest daily increases were observed between days 15 and 16 (1.01 mm) and days 21 and 22 (1.03 mm), while the average daily increment for the remaining days was between 1.5 mm and 2.5 mm. Major changes in external appearance occurred on days 13, 14, 17 and 20 of gestation. Those features which could be observed externally were described for each of the days during the period studied. Late prenatal development in the gerbil resembles that of other myomorph rodents but proceeds at a slower rate than in other species such as the mouse or hamster. This slower rate of development may be of value when precise timing of drug administration and recovery of embryos is necessary.