THEORIES OF CONTROL OF NERVE OUTGROWTHS 151 



determined so that it always points toward the region of increasing concen- 

 tration of the chemical substance from the limb. This phenomenon is called 

 chemotaxis, and it is a general biological phenomenon in which there is an 

 orientation of cells in relation to chemical agents. 



The electrical hypothesis depends on the principle of galvanotaxis, or the 

 orientation of cells in an electric current, and proceeds from the idea that a 

 growing structure is usually negative to the rest of the organism (Fig. 88, B) . 

 Thus if one electrode is placed on the growing limb and another electrode 

 on the back, a current would be expected to flow through a galvanometer 

 from the back to the limb. Accordingly, it is assumed, then, that externally 

 a current flows from the positive region of the back to the negative region of 

 the limb and that there is an internal return, as in a battery. The nerve fiber 

 in coming out of the spinal cord might possibly grow along the lines of flow 

 of an electric current. And, indeed, it is found that an electric current will 

 orient certain cells and cause them to migrate either to the positive or nega- 

 tive pole. 



The mechanical theory states that channels are set up between the limb 

 and the spinal cord in the following way. The colloidal particles in the body 

 fluids of the embryo are oriented with their long axes parallel (Fig. 88 C). 

 And the cone of growth, in emerging from the neuroblast in the cord or the 

 ganglia, gets caught between these oriented particles and is mechanically 

 directed to the limb. The evidence for this theory comes from some tissue- 

 culture experiments in which the outgrowth of nerve fibers in an unorganized 

 medium is compared with outgrowth in a medium in which the particles 

 are oriented. Nerve fibers grow out in all directions from an explant in the 

 unorganized or unoriented medium (Fig. 89, A). If the particles in the 

 medium are oriented in the direction of the arrow (Fig. 89, B) the nerve 

 fibers grow in a parallel fashion from this explant. The orientation of the 

 particles in the medium is brought about by stretching the coagulating clot 

 as it changes from a fluid to a gel. If it is stretched at that time, the long, 

 narrow particles forming in the clot tend to orient with their long axes 

 parallel. Finally, if two cultures of neuroblasts are placed on the same slide, 

 it is found that the growth of these cultures orients the medium in between 

 them so that when the nerve fibers begin to grow out they grow in parallel 

 fashion toward each other (Fig. 89, C) . The assumption is that the growing 

 limb bud in the embryo produces a tension or stretching in the medium 

 between the limb and the spinal cord and in this way orients the long, narrow 

 colloidal particles in parallel lines. The process would be somewhat as 



