96 



NERVOUS SYSTEM 



(His '), in which the trabeculse have, in a general way, a radiating disposition 

 and the oval nuclei, which are distributed throughout the whole thickness of the 

 wall, have a vertical direction (fig. 131A). On both the inner and outer surfaces th( 

 trabeculse fuse to form a limiting membrane (membrana limitans internet, and 

 externa). The nuclei now become densely crowded in the central part of the 

 thickening wall of the tube, and on the outer aspect a layer appears in which thei 

 are no nuclei. It is crossed by delicate, radially disposed, protoplasmic strands, 

 which break up into a system of anastomosing threads. The original epitheliui 

 has thus been converted into a protoplasmic framework (myelospongium), whicl 

 has a columnar and radial disposition and shows three layers : (1) An inner germiv 

 zone ; (2) a middle nuclear mantle zone ; and (3) an outer nuclear-free reticular 01 

 boundary zone. The columns of the framework are not separate, long-drawn-out 

 epithelial cells, but the radial trabeculse of a continuous syncytial network, which 



are more accentuated than the side 

 branches (Hardesty 2 ). In the germinal 

 zone the dividing nuclei are separated 

 by delicately striated columns of 

 protoplasm which broaden out and 

 fuse into the internal .limiting mem- 

 brane (fig. 131B). This appearance 

 due to the persistence in this inner 

 layer of the second stage of the 

 epithelium, when the cell- outlines 

 have disappeared and it consists of 

 columns of protoplasm. The inner 

 zone is afterwards, when the nuclei 

 have "ceased to divide, resolved into 

 the ependymal lining of the canal, 

 and each ependymal cell is continued, 

 through the thickness of the wall of 

 the tube, into a fibre which represents 

 a radial strand of the myelospongium 

 (fig. 132). During the transformation 

 of the neural epithelium into the 

 myelospongium nuclear proliferation 

 has been very active, and the crowded 

 nuclei in the mantle zone (during the 

 third stage) are obviously the products 

 of the divisions in the germinal zone. 

 Among them there can now be de- 

 tected certain nuclei which appear to 

 belong to pear-shaped cells with long tapering processes. These are the future 

 nerve-cells, and were named by His neuroblasts in contradistinction to the cells of 

 the framework which he termed spongioblasts. It is generally admitted, the exist- 

 ence of two categories of cells in the early stages being allowed, that both 

 spongioblasts and neuroblasts spring from the multiplying ectoderm-cells; and 

 further it is believed that the neuroglia-elements of later stages are also of ectodermic 

 origin, though mesodermic elements i.e. common connective-tissue cells are 

 necessarily introduced with the blood-vessels which invade the epithelium, The 

 appearances described have been interpreted in two different senses. According to 

 the interpretation hitherto almost universally accepted and due to His, each 

 neuroblast has a separate and isolated protoplasmic body from which a single 



FIG. 132. SECTION OF THE SPINAL CORD TO SHOW 

 THE ARRANGEMENT OF THE EPENDYMAL AKD 

 GLIAL FIBRES OF THE DEVELOPING CORD. 

 (Lenhossek.) 



1 Die Entwickelung des menschliclieii Gehirns wabrend der ersten Monate ; Leipzig, 1904. 



2 Amer. Jour, of Anat. ii. 1904. 



