10. The _Fore-brain_ soon buds off from each side a small vesicle. This is the optic vesicle, the first rudiment of what subsequently becomes optic nerve and retina. At this period it is simply a vesicle with a hollow stem, the cavity being continuous with the cavity of the cerebral vesicle, and the walls continuous with the cerebral wall. It thus appears that the retina and optic “nerve” are primitive portions of the brain--a detached segment of the general centre, identical in structure with the cerebral vesicle, and not unlike in form. A cup-like depression quickly forms the optic vesicle into an inner and an outer fold. The inner or concave fold becomes the retina, and the outer or convex fold (that nearest to the brain) becomes its choroid membrane. On the fourth day of incubation the retina of the chick is composed of spindle-shaped cells, all alike. On the seventh day there is a differentiation into layers, one of which on the eighth day is granular; on the tenth two are granular; and on the thirteenth ganglionic cells appear. Some of the cells have elongated into radial fibres (known as Müller’s fibres); and with the appearance of rods and cones the normal retinal elements are complete. 11. The researches of Foster and Balfour[84] confirm the statement that all the different parts of the retina (whether nervous or connective) are derived from one and the same layer of embryonic cells, which originally formed a portion of the first cerebral vesicle. 12. Meanwhile the hollow stem of this optic vesicle begins to develop fibres amidst the nuclei of its walls. The “optic nerve” arises: it is still hollow; and in birds remains so through life. The fibres as they are developed _grow forwards towards the retina_, and spread over its internal surface. _They also grow forwards towards the brain_, and spread over its substance; but it is _not_, as might be supposed, and is generally believed, with the cerebral hemispheres (or that portion of the Fore-brain from which these are derived), but with the Middle-brain (which becomes the _corpora quadrigemina_), that the optic fibres are in connection.[85] 13. This will be understood when the further development is traced. The Fore-brain, after budding off the optic vesicles, buds off two larger vesicles--the future cerebral hemispheres. This is noticeable on the second day of incubation, and by the third day each vesicle is as large as the whole of the original Fore-brain. Their development is essentially like that of the optic vesicles; both as to the cellular and the fibrous elements. The _convolutions_, _corpus callosum_, _nucleus lentiformis_, and _corpora striata_ are then indicated. Meanwhile, that which originally was the Fore-brain has lapsed into the secondary rank as Intermediate-brain (_Zwischenhirn_), and becomes the parts surrounding the third ventricle, namely, the _thalami_, _corpora candicantia_, _infundibulum_, and what is called the “posterior perforated substance.”