NEEVOTJS SYSTEM OF VEETEBEATA. 



501 



wliicli pass into the prisms, and tlie free surface of the electric 

 plates are directed dorsally. Five large nerve-trunks pass to the 

 organ ; these rami electrici belong to different cranial nerves, but 

 principally to the vagus ; they are distributed between the pi-isms. 



In the other Electric Fishes these organs agree with what has 

 been described, so far as their more minute characters are concerned, 

 but they differ in position, and in the characters of the alveoli. In the 

 Electric Eel, for example, the organs lie in the caudal region of the 

 body, just below the external integument. In Malapterurus the 

 organ extends over the whole surface of the body, just below the 

 integument; and in the Mormyri again it is found in the tail. There 

 are corresponding differences with regard to the nerves, so that 

 we may conclude that these organs are morphologically different, 

 notwithstanding their histological and physiological similarity. 



ScHULTZE, M., Zur Kenntniss d. elektr. Org. d.Fisclie. Abh. d. Naturf orsch. Gesellsch. 

 Halle, 1858. 



Nervous System. 



§ 379. 



The central organs of the nervous system are placed above 

 the axis of the spinal chord, in the canal formed by the superior 

 system of arches of the axial skeleton. They consist of symmetrically- 

 arranged nervous masses, which are similar in character throughout, 

 in the Acrania only ; in the Craniota they are differentiated into 

 two large portions, the brain, and the spinal chord. Although 

 the latter has without doubt a great similarity to the ganglionic 

 chain of segmented Invertebrata, it is quite impossible to derive the 

 spinal chord from it ; the central nervous system of the Verte- 

 brata is rather to be regarded as representing the superior, 

 or cerebral, ganglia of the Invertebrata in an extremely 

 high state of development. The earliest rudiment is derived from 

 a differentiation of the ectoderm. The " medullary plate," which is 

 formed in this way in the Invertebrata, does not extend along the whole 



of the rudimentary body, or if it 

 does so at first, it does not keep 

 pace with thegrowth of thebody 

 (Ascidi^) ; in the Vertebrate 

 embryo, however, it is of very 

 nearly the same length as the 

 body, so that the central nerv- 

 ous system extends throughout 

 the whole length of the body. 

 The medullary plate forms 

 a groove by the uprising . of 

 its edges (h;), which are continued into the neighbouring ectoderm 

 (epiblast) (Fig. 278, h) ; this is gradually converted into a closed 



Fig. 278. Diagrammatic section through 

 the embryo of the Fowl (end of the first 

 day), c/i. Notochord. « Primitive vertebra;. 

 sp Lateral plates. m Medullary groove. 

 IV and h Epiblast. d Hypoblast (after 

 Eemak). 



