EFFECT OF THE LIMB ON DIFFERENTIATION OF NEUROBLASTS 153 



Effect of the limb on differentiation 



of neuroblasts 



Leaving for a moment the nature of the factors which determine the 

 outgrowth of nerves, let us see what effect the limb has upon the differentia- 

 tion of the nerves after they make contact with it. We know from observation 

 that the limb nerves are larger than the other spinal nerves. And the spinal 

 ganglia of the limb nerves are larger than the other spinal ganglia. We may 

 ask whether this enlargement is inherent in the nervous system itself or 

 whether it is caused by the fact that these nerves make contact with the limb. 



The simplest way to distinguish between these two possibilities is to 

 remove the limb on one side and leave it intact on the other. If the increase 

 in the size of the nerves is due to the presence of the limb, one side should 

 show large nerves and the other side small nerves. Thus, in the chick embryo, 

 we may remove the limb bud on one side and leave it intact on the other, and 

 then examine the embryo at some later time. 



The results of such an experiment are shown in Figure 90. The spinal 

 cord in early development shows a differentiation into a motor region, and 

 the outgrowths of the neuroblasts from this region go through the spinal 

 ganglia and innervate the limb. The spinal ganglia contain the cell bodies for 

 the sensory nerves, and the reflex arc is completed through the spinal cord. 

 On the operated side, where the limb has been removed, the motor region of 

 the cord is smaller and the spinal ganglia are smaller. When the limb is 

 absent we find a 35-percent reduction in the number of senory cells in the 

 spinal ganglia and a 30- to 60-percent reduction in the number of motor 

 neurons originating in the spinal cord. There is some influence from the 

 limb, therefore, which stimulates the differentiation of sensory cells in the 

 sensory ganglia and of motor cells in the motor region of the cord. 



The stimulation by one structure of cell differentiation in another struc- 

 ture is called embryonic induction. In the case just considered we find that the 

 limb induces the differentiation of the cells in the nervous system. Remove 

 the limb and some of these cells fail to differentiate. But why do any differen- 

 tiate? Some will differentiate because the limb nerves, after the limb is 

 removed, will innervate muscles of the back. Thus the nerves are present 

 even if they do not go to the limb muscles. Indeed there is a quantitative 

 relation between the amount of muscle innervated and the number of nerve 

 cells which differentiate. 



