A study of diencephalic development in the albino rat

His (1893) used the sulcus limitans to separate the alar sensory plate from the basal motor plate in the diencephalon. He further subdivided this region into epithalamus, thalamus, metathalamus, and hypothalamus; and, since he ended the sulcus limitans in the optic recess, the bulk of the hypothalamus was basal plate derivative and the rest of the diencephalon was alar plate derivative. This classification did not gain general acceptance for the diencephalon because there is considerable disagreement as to the cranial termination of the sulcus limitans, and there is no simple division of the forebrain into dorsal sensory and ventral motor plates. On the basis of studies on amphibian embryos, Herrick ('10) proposed a more widely accepted diencephalic classification when he divided this region into four longitudinal columns by means of three ventricular sulci and a roof and floor plate. The four columns he named the epithalamus, dorsal thalamus, ventral thalamus, and hypothalamus, and the delimiting sulci he named the sulci diencephalicus dorsalis, medius, and ventralis. Herrick did not reject the concept of the sulcus limitans but stated that it ended in the optic recess and that the sulcus diencephalicus medius was an offshoot from it. Herrick then stated that the hypothalamus was an olfacto-visceral correlation center, that the ventral thalamus was a center for somatic musculature, that the dorsal thalamus was a receptive center for somatic sensory impressions, and that the epithalamus was a correlation center for olfactooptic and olfacto-somesthetic impulses. The morphologic part of Herrick's proposal is generally accepted. Indeed, Ariens Kappers, Huber, and Crosby ('33) in their comparative neuroanatomy text state that all vertebrates with the exception of a few cyclostomes can be fitted into Herricks schema of diencephalic development. However, since that time, only Rose ('42) has demonstrated a more specific attribute of any of these large regions by showing that after cortical removal the adult derivatives of the dorsal thalamus degenerated while the adult derivatives of the epithalamus were unchanged. Herricks diencephalic units have not been successfully correlated with the neuromeric bulges of the embryonic brain. The neuromeres are, at least in the brain stem and spinal cord, segmental aspects of the nervous system (Hill, 1895; Johnston, '05, '09; Herrick, '17; Bergquist and Kdlen, '54) for they are associated with the segmental nervous innervation of the myotomes. However, though neuromeres form in those parts of the brain above the cranial nerves, their absence of connections with the segmental nerves has led some to refer to this part of the brain as suprasegmental (Meyer and Housmann, '22; Rose, '42) while others (Locy, 1895; Johnston, '05) felt that the neuromeres alone were adequate evidence for the segmental derivation of the forebrain. Since the neuromeres appear and disappear so early in development, the adult representatives of these structures are rather poorly understood. Locy (1895) related the first neuromere to the olfactory forebrain, the second to the optic system and the surrounding forebrain, and the third to the pineal. Hill (1899) essentially confirmed Locy's formulation in his paper on the salmon, trout, and chick. Johnston ('05) and Bergquist and KSlen ('54) felt the evaginating optic system developed from a separate neuromere so that neuromere I -'This work was supported in part by A.E. C. rant At(30-11-1454, and by U.S.P.H. S. training grant 2E406.