CHAP. X.] TlliJ DEVELOPMENT OF THE VAT. 361 



modes. The dura mater is formed from the inner surface of the 

 dorsal plates external to the cells which become the nervous centres. 

 The 2)ia mater and the arachnoid, on the contrary, are formed by 

 transformation of the outer layer of the i)rimitive brain mass itself. 

 Thus no part of these tissues, whether choroid plexuses or what 

 not, extend and grow into fossa3 and cavities of the brain, but they 

 actually arise and are first formed there where they ultimately 

 exist. 



The CRANIAL NERVES arise as four small opaque pear-shaped 

 masses of nervous tissue, which grow from the epiblast on each 

 side of the hind brain in front of the protovcrtebrae on each side. 

 Of these four pear-shaped masses (the wider ends of which are 

 directed inwards to the axis of the body), two are placed in front 

 of and two behind the auditory vesicle. The first of these masses 

 becomes the fifth nerve, and bifurcates, one branch becoming the 

 ophthalmic nerve, the other the 2ncl and 3rd branches. The 

 second mass becomes the facial nerve. The third mass becomes the 

 glosso-pharyngeal, and the fourth the pncumogastric. 



The DEVELOPMENT OF THE EYE is brouglit about by the con- 

 currence of three difierent processes or growths. 1. An outgrowth 

 from the brain. 2. An ingrowth from the skin. 3. An upgrowth 

 of the mesoblastic tissue which surrounds the developing eyeball, 

 into a certain part of its interior. 



The outgrowth of the encephalon is in the form of a hollow 

 process of the fore-brain, containing a prolongation of its cavity, 

 the future third ventricle — ^just as the olfactory lobe contains at first 

 a prolongation of the prosencephalic cavity. This ocular outgrowth 

 is called the primary optic vesicle. It ultimately developes into a 

 narrow stalk and an anterior distal expansion. The stalks of the 

 two primary optic vesicles are at first placed close to the junction, 

 on each side, of the cerebral vesicle with the vesicle of the third 

 ventricle. Each is at first disconnected with the other altogether, 

 but little by little the root of each extends over to the opposite side 

 of the brain, while their fibres, where they come thus to cross, inter- 

 mix together, and so a chiasma is formed. The distal expansion of 

 each primary optic vesicle gets (by apposition with other structures 

 to be shortly described) doubled in upon itself, so as to become 

 cup-shaped, and to present a concavity forwards and outwards, the 

 secondary optic reside, or optic cup, while by degrees its cavity be- 

 comes altogether obliterated, as also that of its stalk. 



Meanwhile a depression of the external cuticle has indicated ex- 

 ternally the place of the future eye. This depression deepens while 

 its lips approximate till it forms a closed sac lined by the epiblastic 

 epithelium which coated the invagination. This sac becomes applied 

 to the anterior cup-shaped surface of the optic cerebral outgrowth. 

 The walls of the sac then thicken, especially behind, and obliterate 

 its cavity. Its posterior portion certainly becomes the lens, and its 

 anterior portion either also becomes part of the lens, or is trans- 

 formed into the aqueous humour which, if it is not thus formed, is 



