Formation of the leg neuromuscular system in embryos of the cockroach, Periplaneta americana: I. Light microscope studies

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Abstract
Embryos of the cockroach, Periplaneta americana, were examined each day of their 32‐day developmental period. The interval between the first detection of leg rudiments and hatching was divided into 18 stages defined by a characteristic external morphology. Embryos of stages 1, 2, 3, 6, and 8, corresponding approximately to 6½, 7½, 9¼, 11, and 15 days of age, respectively, were examined in detail for changes in external leg morphology, development of thoracic ganglia and leg muscles, and the formation of connections between the two. These observations were based on a light microscopic analysis of serial sections through three different planes. A proximal distal gradient of muscle development within the leg was observed, the cellular events of which were described. The formation of connections between neurons in the thoracic ganglia and cells in the leg was inferred from observations of the extent of axon growth and the state of differentiation of the cells. The innervation of the coxal muscles may occur as early as stage 6. The role of pioneer neurons, sheath cell and body epithelium in guiding axon growth is discussed. The information in this report will serve as a guide in subsequent more detailed electron microscopic studies.