36 HUMAN EMBKYOLOGY. 



assume a spherical form. These spherical cells have large deeply staining nuclei, 

 and they are termed germinal cells. 



For many years it was believed that the germinal cells were the predecessors 

 of the primitive nerve elements or neuroblasts, and that the remaining cells, called 

 spongioblasts, became transformed into the reticular sustentacular tissue of the 

 central nervous system. It appears, however, from the results of more recent 

 researches, that some of the descendants of the germinal cells become spongioblasts 

 whilst others become neuroblasts or primitive nerve-cells. Moreover, there 

 appear to be two groups of germinal cells ; the descendants of one group are 

 directly transformed into the ependyrnal or lining cells of the central canal, whilst 

 those of the other group form in the first instance indifferent cells, some of whose 

 descendants become neuroblasts and others spongioblasts. The fate of the cells 

 present before the germinal cells appear, and which do not become germinal cells, is 

 uncertain, but they probably take part in the formation of the spongioblastic tissue. 



It is believed, therefore, that all the nerve- cells are the descendants of the 

 germinal cells, and that the spongioblasts which become developed into the cells 

 of the neuroglia or sustentacular reticulum are derived partly from the non- 

 germinal cells of the primitive neural tube and, partly, they are descendants of 

 the germinal cells. 



As differentiation proceeds three layers and two membranes are gradually 

 defined in the walls of the neural tube : (1) a central layer of columnar ependyma 

 cells immediately surrounding the central canal ; (2) an intermediate or mantle 

 layer consisting of neuroblasts and their processes, the nerve-fibres, intermingled 

 with spongioblasts ; (3) a peripheral reticular layer consisting, at first, of processes 

 of the bodies of the spongioblasts. The membranes are an external limiting 

 membrane, surrounding the exterior of the tube, formed by the fused outer ends 

 of the spongioblastic cells, and an internal limiting membrane bounding the 

 central canal and continuous with the inner ends of the ependyma cells. Through- 

 out the whole of the spinal medulla and the brain, the ependyma cells become 

 transformed into the columnar ciliated cells which line the cavities of the adult 

 brain and spinal medulla. The mantle layer becomes converted into the gray 

 matter of the adult central nervous system. 



The peripheral reticular layer, in the spinal region, becomes permeated by 

 nerve-fibres, which are merely processes of the nerve-cells, and it is thus converted 

 into the white matter of the adult spinal medulla. In the brain region it is 

 either transformed in the same way into white matter, or it remains in a more 

 rudimentary condition as a thin peripheral layer of neuroglia on the surface 

 of the gray matter. On the other hand, in the brain region white matter is 

 formed internal to the gray matter by the growth of nerve-fibres which insinuate 

 themselves between the mantle layer externally and the bodies of the ependyma 

 cells internally. 



As the histological differentiation of the walls of the neural tube is proceeding 

 each lateral wall is divided into a dorsal part, the alar lamina, and a ventral part, 

 the basal lamina, by a sulcus-like dilatation of the central canal called the sulcus 

 limitans. After the limiting sulci are formed the parts of the walls of the neural 

 tube are a roof-plate, a floor-plate, and two lateral walls, each of which consists of 

 an alar lamina, essentially sensory in function, and a basal lamina, essentially motor 

 in function (Fig. 44). 



The Fate of the Cavities of the Primitive Brain. The cavity of the spinal 

 portion of the primitive neural tube becomes the central canal of the spinal 

 medulla of the adult. The cavities of the primitive brain vesicles are transformed 

 into the ventricles, foramina, and aqueduct of the adult brain. The cavities of 

 the telencephalic divisions of the secondary fore-brain become the right and left 

 lateral ventricles of the adult brain. The cavity of the undivided portion of the 

 secondary fore-brain vesicle, together with the cavity of the primary fore-brain, 

 become the third ventricle or cavity of the diencephalon, and the apertures of 

 communication between the third ventricle and the cerebral hemispheres are 

 the interventricular foramina (O.T. foramina of Monro). 



The cavity of the hind-brain vesicle becomes the fourth ventricle, and the 



