122 DEVELOPMENT. 



branes extend through them and the pia mater passes into the cerebral ven- 

 tricles. 



The Nerves. The nerves are developed, like the rest of the nervous system, 

 from epiblast. The spinal nerves are developed as follows : Close to the point 

 of involution of the epiblast in the median line that is to say, in the angle of 

 junction of the neural and general epiblast a cellular swelling constituting the 

 neural crest appears, and forms a continuous ridge on the dorsal aspect of the 

 neural canal. On this crest enlargements occur, corresponding with the middle 

 of each protovertebral segment. These grow downward between the neural canal 

 and the mesoblastic tissue forming the protovertebne, and occupy a position on the 

 lateral wall of the canal. These enlargements are the rudiments of the ganglion 

 of the posterior root ; they are at first attached to the neural crest from which they 

 spring, but subsequently this attachment becomes lost, and they then form isolated 

 masses on either side of the neural canal, which now contains the rudimentary 

 cord. They consist of oval cells, from either end of which a process eventually 

 springs ; one, passing centrally, grows into the embryonic cord and constitutes the 

 posterior root of the nerve ; the other, growing peripherally, joins the fibres of the 

 anterior root to form the spinal nerve. 



The anterior root is, according to the researches of His, a direct outgrowth 

 of certain cells which are found in the rudimentary cord, and which are named 

 neuroblasts. These cells, like those mentioned above, are oval, and have a pro- 

 longation directed outward toward the surface of the cord. These processes 

 pass out of the cord in bundles and penetrate the mesoblast and join with fibres 

 of the posterior root, and from the point of union the nerve grows toward ita 

 peripheral termination. 



Most of the cranial nerves are developed in the same manner as the posterior 

 roots of the spinal nerves. That is to say, the neural crest, developed from the 

 epiblast, is continued onward, along the dorsal surface of the cephalic portion of 

 the neural tube, as far as the mid-brain. From this a series of swellings at 

 irregular intervals form the rudimentary ganglia, from the polar cells of which 

 the nerve is formed and its connection with the brain established. This appears 

 to be the case with the sensory portion of the fifth, the portion of the facial con- 

 nected with the geniculate ganglion, the auditory and the sensory portions of 

 the glossopharyngeal and pneumogastric. The motor portions of the mixed nerves 

 and the third, fourth, sixth, spinal accessory and hypoglossal arise like the 

 anterior roots of the spinal nerves from neuroblasts in the floor of the aqueduct 

 of Sylvius and of the fourth ventricle. 



The olfactory tract and bulb is a protrusion of the antero-ventral part of each 

 cerebral hemisphere. This protrusion comes in contact with the thickened epi- 

 blast of the olfactory area (see page 125), from which neuroblastic cells, which 

 are formed within the area, pass out and form a ganglion between the area and 

 the olfactory bulb. From this ganglion cell-processes grow centripetally to form 

 the nerve-roots, and centrifugally to form the olfactory nerves which ramify on the 

 Schneiderian membrane. 



The optic nerve arises in a manner somewhat different from any of the other 

 cranial nerves. It will be considered in connection with the development of 

 the eye. 



The sympathetic nerves are probably developed as outgrowths from the 

 ganglia of the spinal and cranial nerves. 



Development of the Eye. The nervous elements and non-vascular parts of 

 the eye are formed from the epiblast, and the vascular portions from the meso- 

 blast ; but the method of development is somewhat complicated. The essential 

 portion of the eye i. e. the retina and the parts immediately connected with it 

 is an outgrowth from the rudimentary brain (primitive ocular vesicle), and this 

 outgrowth is met by an ingrowth from the common epidermic or corneous layer 

 of the epiblast, out of which the lens and the conjunctival and cornea! epithelium 

 are developed. 



