154 



EMBRYOLOGY 



SPINAL CORD 



LIMB BUD 

 REMOVED 



NEURAL CRESTS 



LIMB BUD 



SMALLER SPINAL 

 GANGLION 



SPINAL GANGLION 



LIMB NERVE 



Fig. 90. The influence of the limb on the differentiation of sensory cells in 

 the spinal ganglion and of motor cells in the motor region of the spinal cord. 

 Top: A left limb bud has been removed from a chick embryo before the 

 neuroblasts have begun to differentiate. Bottom: The spinal cord on the left side 

 has fewer motor cells. The spinal ganglion on the operated side contains fewer 

 sensory cells. In the absence of the limb many neuroblasts which normally 

 would have differentiated into neurons have failed to do so. How is the influence 

 from the limb transmitted to the spinal ganglion and spinal cord ? 



The conclusions regarding the control of the differentiation of the motor 

 region of the spinal cord and sensory spinal ganglia were confirmed by the 

 results of transplanting a limb to a new position. In such an experiment the 

 transplanted limb comes to lie opposite new nerves and is innervated by a 

 new set of nerves. As a result these nerves undergo an increase in the number 

 of fibers, so that they approximate the normal number in the limb region. 



Differentiation within the spinal cord 



If the limb controls in some way the outgrowth of the nerve and also to 

 some extent the differentiation of the parts of the spinal cord opposite it, we 

 still have the problem of the differentiation of the various segments of the 



