1180 



EMBRYOLOGY. 



as consisting of two sets : (1) those which arise as outgrowths from neuroblasts 

 situated in the brain, similar to the mode of origin of the anterior spinal nerve- 

 roots ; (2) those which arise from ganglionic rudiments situated outside the brain 

 and derived from the neural crest ; from the neuroblasts of these ganglionic rudi- 

 ments one process grows into the brain and the other outward toward the periph- 

 ery, similar to the arrangement which exists regarding the posterior spinal 

 nerve-roots. To the first group belong the third, fourth, sixth, seventh, eleventh, 

 and twelfth nerves, together with the motor roots of the fifth, ninth, and tenth. 

 To the second group belong the eighth and the sensory roots of the fifth, ninth, 

 and tenth. While, however, the anterior spinal nerve-roots arise in one series 

 from the ventral part of the cord, the cranial motor fibres arise by two sets of 

 roots, ventral and lateral ; the former include the roots of the sixth and twelfth 

 and probably those of the third and fourth, the latter embrace the spinal accessory 

 and the motor roots of the fifth, seventh, ninth, and tenth. 



The olfactory lobe, or rhinencephalon, arises toward the end of the fourth 

 week as a protrusion of the antero-ventral part of each cerebral hemisphere 

 (Fig. 726), and extends forward toward the thickened epiblast of the olfactory 

 area (see page 1169). It is subsequently divided by a transverse constriction into 

 two parts : an anterior, which gives rise to the olfactory bulb and tract together 

 with the trigonum .olfactorium, and a posterior, which becomes the peduncle of 

 the corpus callosum and the greater part of the anterior perforated space. Neuro- 

 blastic cells, formed within the olfactory 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 in the olfactory mucous membrane, while the ganglion itself fuses 

 with the olfactory bulb. 



The optic nerve arises as a hollow outgrowth of the brain, which subsequently 

 becomes solid. It will be considered in connection with the development of the eye. 



The sympathetic nerves are developed as outgrowths from the ganglia on the 

 roots of the spinal and cranial nerves. 



For eb rain 



Imagination of epi- 

 blast to form the 

 lens rudiment. 



1'iijmented layer 

 .'of ret inn. 



^Epiblast. 



Nervous part 



of retina. 

 - - Optic stalk. 



FIG. 729. Transverse section of head of chick embryo of forty-eight hours' incubation ; X 55. (From Duval'a 

 Atlas d' Emory ologie.) 



Development of the Eye. The optic nerve and retina are developed as an out- 

 growth from the rudimentary brain, which extends toward the side of the head, 

 and is there met by an ingrowth from the epiblast, out of which the lens and the 

 epithelium of the conjunctiva and cornea are developed. 



The first appearance of the eye consists in a hollow protrusion of the fore- 

 brain ; this is called the primitive optic vesicle. It is at first an open cavity com- 

 municating by a hollow stalk with that of the cerebral vesicle. As it is prolonged 

 forward, the epiblast lying immediately over it becomes thickened, and then 

 forms a depression which gradually encroaches on the most prominent part 

 of the primitive ocular vesicle ; this in its turn appears to recede before it, 

 so as to become at first depressed and then inverted in the manner indicated 



