THE ORGANS OF THE OUTER GERM-LAYER. 463 



SUMMARY. 

 Central Servous System. 



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) Ainphioxus, (6) Petromyzon, Teleosts, (c) the re- 

 maining Vertebrates. 



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 longitudinal 

 fissures, 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 (cephalic 

 flexure, pontal flexure, nuchal flexure). The cephalic or parietal 

 protuberance at the surface of the embryo corresponds to the cephalic 

 flexure, the nuchal protuberance to the nuchal flexure. 



