Fig. 1. Mitotically competent cells within the myotome first appear at the extreme edges and later scatter throughout the entire muscle mass. (A) Frontal section through a cervical level of the axis of a 35-somite stage quail embryo showing the presence of thymidine-positive nuclei predominantly at the rostral and caudal extremities of the desmin-positive myotome. (B) Mitotically active cells in the nascent muscle of an E4 embryo are scattered throughout the entire structure. In both cases, short pulses of radiolabeled thymidine were delivered followed by immediate fixation thus reflecting intrinsic cell proliferation. D, dermis; M, myotome, S, sclerotome. Scale bar: 30 µm in A; 70 µm in B.