Possible influences upon patterns of neurogenesis expressed in vitro were examined quantitatively by the use of microfragment cultures of embryonic day 10 mouse neural tube. Crude extracts were prepared either from whole embryos (day 13 or 15 of gestation) or from embryonic brains (day 18 of gestation) and added to the culture medium for the first 10 days of culturing. Neuronal outgrowth zones surrounding individual microfragments were reduced in area (indicating restricted neuronal migration) and in number of neurons present (indicating restricted production of neurons) following treatment with either of the extracts. The severity of reductions observed were related to the developmental age of embryonic tissue used for preparing the extract, as greatest reduction resulted from addition of embryonic day 18 brain extracts and to concentration employed, higher doses further restricting neuronal outgrowth. By increasing the concentrations of extract the proportional number of large-sized neurons forming the outgrowth zones became greater relative to the small neuron contribution, indicating an enhanced survival for this neuronal population. The formation and migration of astroglial precursor cells was not affected by the addition of any of the extracts. The number of neurons remaining within the original portions of neural tube microfragments was not significantly altered following culturing in the presence of embryonic extract. This suggested that the reduction in neuron number in the outgrowth zone actually reflected a decreased neuron production and was not simply the result of a retention of neurons within the remaining portion of the microfragment. The results suggest the presence of substances within mouse embryos that have regulatory effects on aspects of development of the central nervous system. Indications are that survival and maturation of postmitotic neuroblasts are promoted in vitro while the formation of additional neuronal progenitor cells may be partially inhibited by the addition of embryonic mouse extracts to the medium. We propose that an endogenous negative feedback mechanism may be involved in the coordination of patterns of neurogenesis.