2 6o TEE POPULAR SCIENCE MONTHLY 



nervous materials. Since the nerve-tube from which the central 

 nervous organs in vertebrates are developed is infolded ectoderm, it 

 follows that the inner surface of the tube represents a portion of the 

 outer surface of the animal. This inner surface even in the adult 

 central nervous system is always covered by an epithelium as the 

 exterior of the animal is, and the nervous materials which surround it 

 are related to this epithelium in a characteristic way. This relation 

 can be most easily seen in any transverse section of the spinal cord. 

 Beginning at the central canal of such a section (Fig. 5) and proceed- 



17^0® 0® ' 



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Fig. 5. Transverse Section op the Spinal Cord op a Vertebrate (Sala- 

 mander), c, central canal; e, epidermis; g, gray substance composed of ganglion- 

 •cells and neuropile ; w, white substance or nerve-fibers. 



ing through the substance of the cord to the opposite face, one passes 

 first an epithelial layer, then gray substances composed of nerve-cells, 

 neuropile, etc., and finally white substance made up of nerve-fibers. 

 Trecisely this sequence is met with in the central nervous system of any 

 primitive invertebrate such as Segalion, where, as already pointed out, 

 in passing through the thickness of the central nervous organ from the 

 exterior to the interior one meets first external epithelium, then gan- 

 glion-cells and fibrillar corresponding to the gray substance of verte- 

 brates, and finally nerve-fibers corresponding to the white substance of 

 these animals. Thus the nervous materials of the vertebrate spinal 

 cord are distributed through that structure on a plan similar to that 

 found in invertebrates, and this plan, though considerably modified, also 

 holds good for the vertebrate brain. So far as these particulars are 

 concerned, the vertebrate central nervous system differs from that of 

 the higher invertebrates chiefly in that in separating from the integu- 

 ment it has carried with it its epithelial mother-tissue instead of leaving 

 this tissue behind. 



Not only are the materials of the vertebrate central organs distrib- 

 uted on a plan that is best understood from the standpoint of the inver- 

 tebrates, but the primary neurones of vertebrates are also most clearly 

 interpreted from this point of view. The primary motor neurones of 



