144 EMBRYOLOGY 



Fig. 84. The optic stalk directs the growth of the 

 nerve fibers from the optic cup to the brain. Neuro- 

 blasts that give rise to the optic nerve are in the 

 inner layer of the optic cup. The cone of growth 

 of a neuroblast migrates along the inner surface of 

 the optic cup to an opening, the choroid fissure, 

 thence along and within the optic stalk back to the 

 brain. 



CHOROID FISSURE 



Soon after the cells have migrated from the main mass, the large, rounded 

 neuroblasts can be distinguished in the tissue culture. After a few days these 

 neuroblasts begin to show outgrowths from one side of the cell. The out- 

 growths are amoeboid in nature and trail off into very fine filaments of 

 protoplasm. If we follow one cell, we find that within a few days the ameboid 

 outgrowth has advanced a millimeter or two. It remains connected with the 

 main body of the cell by a thin strand of protoplasm which becomes the nerve 

 fiber. The amoeboid outgrowth is called the cone of growth, and it migrates 

 along the bottom of the cover slip or any other solid object just as an amoeba 

 would. 



The migration process continues until the nerve fiber has been spun out 

 into a very long, thin fiber. Such a fiber may show branching and may have 

 two cones of growth. Commonly it follows a very devious pathway through 

 the tissue-culture medium. Thus the outgrowth is not directed along any 

 specific path. It can change its direction. For example, if this outgrowth meets 

 an obstacle — and in tissue culture in a blood clot the solid fibrin would form 

 an obstacle — the cone of growth will be deflected from its random path and 

 crawl along the piece of fibrin. In general, the cone of growth tends to follow 

 solid objects as it progresses. The tendency to move along solid objects is 

 termed stereotropism. 



This property of the outgrowing nerve fiber is expressed in the develop- 

 ment of the optic nerve (Fig. 84). When the optic cup is being formed, it 

 remains attached to the optic stalk, which connects it with the brain. The optic 

 stalk is not the optic nerve; it is merely a connection between the cup and the 

 brain. The optic nerve is formed by a group of fibers which have their origin 

 in cells in the inner layer of the optic cup. These fibers grow back along the 

 optic stalk into the brain. Each neuroblast sends out its own fiber and uses 

 the optic stalk as a solid substrate, literally crawling back into the brain. 



