The development of the nucleus of origin of centrifugal fibers to the retina in the chick

The development of the isthmo‐optic nucleus – the site of origin of centrifugal fibers to the avian retina – has been studied in normal chick embryos and after early complete or partial removal of the optic vesicle. The nucleus arises from the junctional region between the caudal midbrain tegmentum and roof plate at about the sixth or seventh day of incubation and soon forms a compact, oval cell mass midway between the trochlear nucleus and the medial edge of the tectum. Within three or four days it is invaded by afferent fibers from the optic tectum, through the tecto‐isthmal tract, and becomes transformed into a convoluted, but essentially two‐dimensional sheet, by a large scale degeneration of cells, especially in its central part. Extirpation of the optic vesicle has no effect upon the initial proliferation, differentiation or migration of the cells, but the ensuing cellular degeneration is considerably exaggerated, so that after complete removal of the vesicle, the nucleus shows a total cell loss by the nineteenth day of incubation. The tecto‐isthmal fibers disappear at, or just before, the period of maximal cell death; the centrifugal fibers from the nucleus (the isthmo‐optic tract) degenerate at a slightly later stage. Large, but incomplete, lesions of the vesicle before Stage 13 result in considerable cell losss throughout the nucleus; smaller lesions at these stages result in only a slight cellular hypoplasia. Incomplete lesions at Stage 13, or later, usually result in a complete cell loss in one or other topographical portion of the nucleus, the remaining parts being unaffected.