A chronological, quantitative light microscopic investigation on the histogenic organisation and behaviour of the trigeminal-ganglion was carried out in the ontogeny of the chick. Embryos of 24 chicks at embryonic days 6 - 18 (E6 - E18), on the day of hatching (H) and adult (A) were processed. Paraffin sections of 8 to 10 microns thick were stained with cresyl-fast-violet. Dark and light cells of various size-categories were observed. There were two periods of active proliferation: one on Embryonic Day 8 (E8) and another on E18. An active reduction in the total number of cells occurred during E10-E13 and during post-hatching period. Few light-cells started to appear on E8, continued to increase in number to reach its highest peak on E13, while the number of dark cells have declined. There was a further fluctuation in the relative numbers of the dark and light cells during the period between E15 - E18. The day of hatching marked a second peak of light cells and decline of dark cells. There was a tendency of increased proliferative activity, growth, and maturation of neurones as from E18 with the appearance of larger cell types. The periods of accelerated degeneration of ganglion cells may correspond to the period of active establishment of functional connections. Cell-death might have occurred due either to a failure of these projection-fibres to reach the target-fields or to a failure of making a functional connection. The reduction in the number of light-cells might be associated with the removal of these cells due to the increased phagocytic activity. On the basis of the fluctuation in the number of dark and light cells, it is assumed that the dark-cells represent a group of active cells while light-cells are either resting, inactive or dying cells. This assumption need to be confirmed by physiological and immunocytochemical studies.