CH. XVII. J 



NERVE-CELLS. 



191 





one in origin. Like the rest of the nervous system, it originates 

 from the outermost layer of the embryo, the epiblast. All true 

 connective tissues are mesoblastic. 



Chemically it is very different from connective tissues. It 

 consists of an insoluble material called neuro-keratin, or nerve- 

 horn, similar to the horny substance keratin which is found in 

 the surface layers of the epidermis. 



Structure of Nerve-Cells. 



Nerve-cells differ a good deal both in shape and size. 



In the early embryonic condition, the future nerve-cell is a 



N.S. 



Fift. rpy. Bipolar nerve-odls. A. From the Gaswriim pin^'lion of the pike (after 

 lli'l'leri. B. From a urinal pinli'Hi <>f a 4J weeks' human embryo (after Bin). 



' . A. lull i litjmi nl ilir mammalian spinal ganglion cell: N.S. nucleated 



ith ; only the nuclei seen in profile, an- represented. T. is the T-shajied 

 junction (after Het/in-.). 



small nucleated mass of protoplasm without processes. As develop- 

 ment progresses branches grow, and by this means it is brought 



