The levels of the neural axis from which parasympathetic and orthosympathetic neurons and adrenomedullary cells are derived under normal developmental conditions were determined in avian embryos by a biological labeling technique. The technique is based on nuclear differences between two species of birds, the chick and the quail. In quail interphase nuclei a part of the chromatin is condensed in large heterochromatic masses associated with nucleolus, while in the chick, DNA is evenly dispersed in the nucleoplasm. These characteristics provide a stable nuclear marker that can be used to study cell migrations and differentiation in chimeric embryos resulting from the association of quail and chick tissues. Isotopic and heterotopic transplantations of quail neural primordium into chick before the outset of neural crest cell migration show that the autonomic ortho- and parasympathetic neuroblasts are not determined to differentiate into cholinergic or adrenergic neurons when they begin to migrate. The neurotransmitter synthesized by crest autonomic neuroblasts depends on the microenvironment in which crest cells become localized at the term of their migration. The splanchnic mesoderm induces presumptive adrenergic cells to become fully differentiated cholinergic neurons.

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Chick biventer cervicis muscle fibres have been studied histochemically. Fast-twitch, focally innervated (alpha) fibres represent 70-80% of the total fibres in this muscles. Two histochemical profiles of slow-tonic multi-innervated (beta) fibres have been observed from embryonic life the adult (three-months) stage. These two slow-tonic types differ in the activity of their histochemically demonstrated myofibrillar ATPase after either acid or alkaline preincubation, and after formalin fixation. Both slow-tonic fibre types have a high oxidative metabolism and are PAS-negative. They are referred as to beta 1 and beta 2R fibre types (slow-tonic oxidative) in an expansion of Ashmore's nomenclature, and compared to avian slow-tonic sub-types that have been described in recent reports. beta 1 and beta 2 fibre types exhibit a similar pattern of innervation. Possible explanations of the origin of histochemical heterogeneity in multiple innervated fibres are discussed.

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