Though muscle characteristics in the second half of gestation closely relate to quantity and quality of postnatal muscle, far fewer studies have examined breed differences in prenatal sheep muscle. This study seeks to better understand anatomic prenatal muscle differences and its underpinning mechanisms between two contrasting sheep breeds. A total of 38 sheep fetuses, including 20 Texel with pronounced muscularity and 18 Ujumqin, an indigenous fat -tailed sheep characterized by their hardiness, fetuses aged at up to five time-points from embryonic day (E) 70-135 of gestation were sacrificed and their organs and muscles dissected at 12 anatomical locations from the right side of each animal. Whole Texel fetuses weighed more especially at E70 and E85 (both 1.7-fold, P < 0.001) with heavier internal organs (P < 0.05), than age-matched Ujumqin fetuses. Texel fetuses also contained more muscle, including aggregate-excised muscles (70% before E100, P < 0.001) and muscle proportion (similar to 0.5%, P < 0.05), larger (similar to 20%) and more total muscle fibers, greater muscle size (20% loin muscle area, P=0.008), and a lower proportion of type I muscle fibers (Texel, 18.85%; Ujumqin, 25.23%) at most anatomical locations. Notably, in Texel fetuses from E70 to E85, sharply decreased myofiber number and significantly augmented fiber size were accompanied by the molecular events resembled those in myoblast fusing of Ujumqin sheep (E85-E100), which indicate this period (E70-E85) is pivotal for Texel myoblast fusing. Altogether, these results suggest that comparing with obese animal, the muscular superiority in lean breed largely establishes at middle gestation, although they share the similar regulatory gene network. The present study provides a novel insight into mammalian myogenesis. (C) 2016 Elsevier B.V. All rights reserved.