148 EMBRYOLOGY 



Here, as can be seen from the Figure 82, there is no visible pathway between 

 the neural tube and the neural crest and the mesoderm of the limb. A mass of 

 mesenchyme and some somites intervene, but there is nothing that would 

 direct the nerve to the limb. 



There are only two possibilities: either the nerve shoots out from the 

 spinal cord in a predirected fashion so that it will hit the limb, or the limb in 

 some way directs the outgrowth of the nerve fiber. One may choose between 

 those two possibilities by considering the results of transplanting the limb 

 to a new position. In the amphibian larva the forelimb is supplied by the 

 third, fourth, and fifth spinal nerves. The location of the spinal nerves along 

 the dorsal side of the embryo is shown in Figure 86. When the limb develops, 

 nerves from the third, fourth, and fifth segments of the spinal cord and 

 spinal ganglia supply it. Now suppose in an early stage of development, 

 before the nerves have had a chance to grow out, we cut out the early limb 

 disk and place it in a new position opposite the sixth, seventh, and eighth 

 spinal segments. Then we can ask whether the third, fourth, and fifth nerves 

 will still innervate this limb or whether it will now be innervated by nerves 

 six, seven, and eight, which are closer to the limb. The fact is that when, by 

 early transplantation, the limb is moved posteriorly into a new position, the 

 third, fourth, and fifth nerves will still turn back, deviate from their normal 

 pathway, and innervate the displaced limb. Therefore, the limb in some way 

 directs the cone of growth of the outgrowing nerve fiber. This phenomenon 

 has been called an attraction of the nerve fiber by the developing limb. 



What is the nature of this attraction? In the first place, will the limb in 

 any position always attract these particular nerves? The answer to this 

 question is clear. There is a limit to the distance over which this attraction 

 is effective. If the limb is placed in a position more posterior than in the first 

 operation — if it is put too far away from its normal site — then it will attract 

 the sixth, seventh, and eighth nerves to itself (Fig. 87) . There is a limit, then, 

 to the distance over which the limb will attract the normal limb nerves. 



The next question we should like to ask is whether this attraction is a 

 very specific one, originating in the limb only, or whether it is an attraction 

 which may result from any growing organ. The experiment which throws 

 light on this question is one in which we take out the forelimb and move it 

 back as we did before and at the same time substitute in its place the olfactory 

 placode. Then we find that the third, fourth, and fifth nerves grow out to 

 the olfactory placode as if it were a limb. In other words, they are attracted 

 by this foreign structure. But they do not penetrate the olfactory placode, 



