DEVELOPMENT OF THE EPENDYMA AND THE NEUROGLIA CELLS.
The ependyma cells maintain in the foetal stage the character of an epithelium and the relations to the membrana limitans externa and interna. In the brain, as in the spinal cord, the ependyma cells extend from the inner to the outer surface of the neural wall, the length of the cells keeping pace with the increase in the tube. The inner portion of the cell, nearer the central canal, retains more the character of a cell-body ependyma cell, while the outer portion gradually diminishes to a delicate fibre, which as an ependyma fibre radially traverses the neural wall. The entire arrangement constitutes the ependyma system or the efiendymium.
On examining this ependymal framework more closely, a distinctive disposition of the ependyma cells is seen in the spinal cord. In a cross-section of the medullary tube of a 3-4 day chick embryo (Fig. 98), we recognize how the ependyma fibres traverse the wall of the tube, at the sides passing almost parallel from the central canal and ventrally and dorsally diverging radially. In conse quence of the coming together of the nucleus-bearing portions of the cells, there appears within the medullary tube, in the vicinity of the central canal, a broad, richly nucleated zone, the inner layer of His or the ependymal of Len hossek. In a general way, this zone corresponds to the later epitheiulm of the central canal. The ependyma fibres of the later anterior commissure present a rough appearance, being heavy and beset with spines ; they also emphasize the already slight meridional disposition of the more laterally' situated ependyma fibres. In a somewhat later stage, the ependyma fibres exhibit varicosities, particularly in their inner portions ; in addition, in their outer portions they undergo a subdivision into several branches, all of which extend to the periphery, where they end in small triangular expansions.
Subsequent stunting of the lateral parts of the ependyma framework is most marked in the spinal cord of the higher forms. In the other parts of the central nervous system, the ependyma cells and fibres retain their embryonal form, even after completed growth. The ependyma cells are, therefore, phylogenetically as well ontogenetically, the oldest cells of the supporting framework, arising directly from the ectoderm cells, or, indeed, being in a modified way these themselves. During
later stages, the elements, particularly the ependyma fibres, are curtailed in varying degrees ; a part of the epen dyma cells later migrate and become the neuroglia cells.
The neuroglia cells arise only after the formation of the ependyma framework. On examining the spinal cord of a ten-day chick, one finds a number of elements which closely semble the ependyma cells, their fibres likewise extending to the periphery and there ending in conical thickenings. They differ from the ependyma cells proper, however, in that their bodies no longer lie at the central canal, hut farther outward. At first such cells are encountered only in the vicinity of the central canal and in meagre number ; later, however, they are more numerous and occur also in the peripheral zone. This is explained by the manner in which the neuroglia cells arise. Originally these cells lie, as do the ependyma cells, at the central canal ; then the cell-bodies migrate from the region of the epithelium, part of the cell-body becoming a thin fibrilla, that later disappears. Small spines and branches appear on the former smooth cell-bodies, as well as similar thorny outgrowths for a short distance along the process stretching from the cell-bodies to the periphery. At first such migrated cells are present only in sparing number ; later, however, their number materially increases and the cells are dis tributed more or less uniformly throughout the entire cross-section of the spinal cord.
This radiating sustentacular apparatus constitutes in man and the higher mammals an embryonal feature. Subsequently, the picture changes. The radial type disappears and the shape of the cells alters. The minute spicules and branches develop very markedly, while the peripherally directed processes atrophy. The cells become the true spider or neuroglia cells. The latter, therefore, pass through various developmental stages ; at first they are ependyma cells, then radial sustenacular cells, from which arise the neurog lia cells.