THE EARLY FORMS OF NERVE CENTRES.
9. In the outermost layer of the germinal membrane of the embryo a groove appears, which deepens as its sides grow upwards, and finally close over and form a canal. This canal is composed of cells all alike. Its foremost extremity soon bulges into three well-marked enlargements, which are then called the primitive cerebral vesicles. The cavities of these vesicles are continuous. Except in position and size, there are no discernible differences in these vesicles, which are known as the Fore-brain, Middle-brain, and Hind-brain.
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.”
14. The Middle-brain, or Second Vesicle, develops the corpora quadrigemina from the roof of its cavity, and the crura cerebri from its floor.
The Hind-brain, or Third Vesicle, divides into two, like the First Vesicle; it buds off the hemispheres of the cerebellum; its cavity forms the fourth ventricle; its walls the medulla oblongata.
15. It thus appears that the primitive membrane forms into a canal, which enlarges at one part into three vesicles, and from these are developed the encephalic structures. The continuity of the walls and cavities of these vesicles is never obliterated throughout the subsequent changes. It is also traceable throughout the medulla spinalis. And microscopic investigation reveals that underneath all the morphological changes the walls of the whole cerebro-spinal axis are composed of similar elements on a similar plan.[86]
16. Two conclusions directly follow from this exposition:—first, that since the structure of the great axis is everywhere similar, the properties must be similar; secondly, that since there is structural continuity, no one part can be called into activity without at the same time more or less exciting that of all the rest.