In the bird embryo, according to the studies of Romer (1927) and Wortham (1948), the muscles within each segment of the leg differentiate from distinct dorsal or ventral mesenchymal masses. Presumably these represent the primitive dorsal extensor and ventral flexor muscle masses. The list below indicates the ontogenetic origin of the avian leg muscles, according to the studies of Romer and Wortham. The individual muscles are discussed in the order in which they are listed below. Dorsal muscles of thigh M. extensor iliotibialis lateralis M. extensor iliofibularis M. extensor iliotibialis anticus M. piriformis M. ambiens M. gluteus profundus M. vastus lateralis M. iliacus M. vastus medialis M. iliotrochantericus medius M. femoritibialis internus M. psoas Ventral muscles of thigh M. flexor cruris lateralis M. adductor superficialis M. flexor cruris medialis M. adductor profundus M. caudofemoralis M. obturator M. flexor ischiofemoralis M. femorocruralis Ventral muscles of shank M. gastrocnemius M. flexor perforatus digiti III M. flexor perforans et perforatus M. flexor perforatus digiti II digiti II M. flexor hallucis longus M. flexor perforans et perforatus M. plantaris digiti III M. flexor digitorum longus M. flexor perforatus digiti IV M. popliteus Dorsal muscles of shank M. peroneus longus M. extensor digitorum longus M. tibialis anticus M. peroneus brevis Dorsal muscles of foot M. extensor hallucis longus M. extensor proprius digiti III M. abductor digiti II M. extensor brevis digiti IV M. extensor brevis digiti III Ventral muscles of foot M. lumbricalis (M. adductor digiti II--not M. abductor digiti IV present) M. flexor hallucis brevis (M. adductor digiti IV--not present) [Illustration: FIG. 12. _Tympanuchus pallidicinctus_ 2L. Lateral view of the superficial muscles of the left leg. × 1.] =_M. Extensor Iliotibialis Lateralis_= (M. iliotibialis), Figs. 12, 13, 20F, G _T. pallidicinctus_ [Illustration: FIG. 13. _Tympanuchus pallidicinctus_ 2L. Medial view of the superficial muscles of the left leg. × 1. Articular capsule shown by concentrically arranged dashes.]