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THE POPULAR SCIENCE MONTHLY. 



out from the brain and spinal cord. The brain and spinal cord, 

 since the days of the celebrated investigations of Karl Ernst von 

 Baer, have been identified as modifications of a single long tube, 

 the so-called medullary tube of embryology. This tube, as the 

 embryo advances, gradually increases in complexity, especially in 

 the region of the head, until it is converted into the brain and 

 spinal cord. The complications which occur may be conveniently 

 grouped under four heads namely, the flexures, the widening of 

 the cavity or its obliteration in a way varying for each region, 

 changes in the thickness of walls, and lastly an extreme differ- 

 entiation of the microscopic organization. Without detailed ex- 

 planation it may be readily conceived that by the varying co- 



Dorsal zone white 

 Ventral zone shaded 



Olfactory Lobe 



Infundibulum. 



operation of these factors great differences arise in the sundry 

 parts of the originally simple medullary tube. On the other hand, 

 in the most fundamental characteristic, the production of nerve 

 fibers, the same principle governs brain and spinal cord alike. 

 There appear very early certain cells, which soon become recog- 

 nizable as young nerve cells (neuroblasts) because of their size 

 and pointed shape; the pointed end now elongates into a very 

 delicate thread, the nerve fiber, which is at first very short but 

 rapidly lengthens almost like a growing root ; the growing fiber 

 takes its course for a certain distance, varying according to cir- 

 cumstances, within the wall of the medullary tube, but ultimately 

 passes outside the tube into the neighboring tissues together with 

 other nerve fibers of similar origin. It must be added that some 



