Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark.

Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    491/704 (page 463)
    Summary. Central Nervous Systenv. 1. The central nervous system is developed out of the thickened region of the outer germ-layer which is designated as the medullary plate. 2. The medullary plate is folded together to form the medullary tube (medullary ridges, medullary groove). 3. The formation of the neural tube exhibits three principal modifications: {a) Amphioxus, (6) Petromyzon, Teleosts, (c) the re- maining Vei'tebrates. 4. The lateral walls of the medullary tube become thickened, whereas the dorsal and ventral walls remain thin; the latter come to occupy the depths of the anterior and posterior longitvidinal lissures, and constitute the commissures of the lateral halves of the spinal cord. 5. The spinal cord at first fills the whole length of the vertebral canal, but it grows more slowly than the latter, and finally terminates at the second lumbar vertebra (explanation of the oblique course of the lumbar and sacral nerves). 6. The part of the neural tube which forms the brain becomes segmented into the three primary cerebral vesicles (primary fore- brain vesicle, mid-brain vesicle, hind-brain vesicle). 7. The lateral walls of the fore-brain vesicle are evaginated to form the optic vesicles, the anterior wall to form the vesicles of the cerebrum. 8. The hind-brain vesicle is divided by constriction into the vesicles of the cerebellum and the medulla. 9. Thus from the three primary brain-vesicles there finally arise five secondary ones arranged in a single series one after the other —(a) cerebral vesicle (that of the hemispheres), (6) between-brain vesicle with the laterally attached optic vesicles, (c) mid-brain vesicle, {cl) vesicle of the cerebellum, (e) vesicle of the medulla oblongata. 10. The originally straight axis uniting the brain-vesicles to one another later becomes at certain places sharply bent, in consequence of which the mutual relations of the vesicles are changed (ceplialic llexure, pontal flexure, nuchal llexure). The cephalic or parietal pi'otuberance at the surface of the embryo corresponds to the cephalic flexure, the nuchal protuberance to the nuchal flexure.