AbstractChick embryo spinal ganglia, cultured with and without he nerve growth factor (NGF), were compared by means of electron microscopy with ganglia maintainedin vivofor an equal period of time. The results confirm an earlier report that NGF maintains and stimulates only the later‐differentiating mesiodorsal neuroblasts. However, although these cells are stimulated to grow neurites four days prematurely, cytoplasmic maturation does not accompany the outgrowth.The more‐differentiated ventrolateral neuroblasts all show some signs of degeneration after culturing with or without NGF. Those cells which appear normal in the light microscope failed to continue development like theirin vivocounterparts and are seen to have myelin figures, marked indentation of the nuclear membrane, and chromatin clumping. Crystalloid structures originally observed by Crain et al. ('64) are demonstrated to be ribosomal crystals and are found only in hyperchromatic degenerated neuroblasts.The satellite cells do not continue to developin vitro. The process of encapsulatin neuroblasts ceases in both the mesiodorsal and ventrolateral areas of the ganglia. The interrelationship of satellite cell and neuroblast develoment is discus