The origin of the vertebrate brain

The need for a "model" of the development of the head and the nervous system is examined. The purpose of such a model is to facilitate the testing of the probability of the correctness of anatomic and developmental ideas from a number of areas. Without such a model this information presents only a jumble of discordant facts. The proposed model suggests the following: 1. The nervous system of amphioxus tells us little, or nothing, about the ancestral vertebrate system. 2. The gap between the neural tube stage, established in the chordate, and the complex central nervous system typical of the vertebrate is partly closed by the identification of two stages. 3. The first of these stages assumes that the original anterior end of the nervous system corresponded with the anterior end of the notochord: thus it was at the anterior end of the present mesencephalon. Outgrowth of the anterior mesencephalic wall, totally sensory in nature, moved the neuropore anteriorly. This outgrowth is correlated with the gradual conversion of a velar mouth to a biting mouth by forward and downward extension of the head. 4. As a result of the anterior and downward expansion of the anterior wall of the mesencephalon, the cephalic flexure and infundibulum were produced. The primitive eye changed its relationship to the brain from attaching anterolateral to laterally; its point of attachment shifted from dorsal to the neuropore to ventral to the neuropore. 5. The nasohypophyseal field in the neural tube ancestor was closely associated with the anterior wall of the brain below the neuropore, and nerve fibers growing from this epithelium extended to the brain below (posterior?) to the optic stalk. With outgrowth, the fibers now entered the brain substance anterior to the optic stalk and dorsal (?) to the neuropore. 6. The second stage, which is well known from the literature, is the outgrowth of the telencephalic lobes from the dorsolateral and anterior wall of the "diencephalon." 7. Among the vertebrates further elaboration of the system can be documented.