The development of the mesencephalic nucleus of the trigeminal nerve in the chick

The normal development of the mesencephalic nucleus of the trigeminal nerve has been studied in a series of chicks from the fourth day of incubation until the eighteenth day post‐hatching. In addition, the time of origin of the cells in the nucleus has been determined by 3 H‐thymidine autoradiography, using a variant of the cumulative labeling method. The fully‐formed nucleus consists of two distinct groups of unipolar cells, a numerically larger medial division within the tectal commissure and a smaller, lateral division within the stratum griseum periventriculare of the optic tectum. The cells in the lateral group are, on the average, larger than those in the medial part of the nucleus at all stages of development. The entire population of neurons is generated between the third and fourth days of incubation (stages 14 to 24) with the cells in the lateral division being formed slightly earlier (stages 14 to 20) than the medially located cells (stages 16 to 24). When the nucleus is numerically complete, at the eighth or ninth day of incubation, it contains between 4,000 and 4,500 cells. Between days 11 and 13 there is a dramatic loss of cells, especially from the caudal part of the nucleus, to a final population of about 1,100 neurons. This normally occurring cell loss seems to occur at the time muscle spindles are being formed, and appears to be due to the inability of the outgrowing processes of the cells to form appropriate peripheral contacts since it is accentuated by the early extirpation of the mesoderm of the mandibular lobe. These experiments also suggest that in the chick the jaw musculature is, in part, innervated bilaterally by the mesencephalic nucleus.