DIFFERENTIATION WITHIN THE SPINAL CORD 



155 



e 7 e s 



53 



NORMAL FUNCTION FROM 0,7,8 



Fig. 91. The substitution of spinal segments 6, 7, and 8 for segments 3, 4, 

 and 5. The presumptive forelimb is not disturbed. When the limb develops 

 it attracts nerves from the transplanted segments, 6, 7, and 8, and shows perfect 

 function. Therefore the spinal segments 6, 7, and 8 of an early embryo have 

 been induced by the new surroundings to develop into segments 3, 4, and 5. 



spinal cord itself — because these various segments of the spinal cord differ 

 to some extent. It has already been shown that the amphibian limb if trans- 

 planted too far posteriorly will attract foreign nerves — the sixth, seventh, 

 and eighth nerves. These nerves increase in size when they make contact with 

 the limb. However, the limb will not function properly with these foreign 

 nerves. There is something different about segments 6, 7, and 8 as compared 

 with segments 3, 4, and 5, for 3, 4, and 5 will give perfect function of the 

 limb, no matter where the limb is placed. Thus there must be regional dif- 

 ferences along the length of the spinal cord. 



How is this differentiation brought about? The principles of early em- 

 bryonic development which we have followed up to this point apply here. 

 There is a labile organization of the spinal cord of the embryo. It is organized 

 so that segments 3, 4, and 5 supply the limb; segments 6, 7, and 8 supply the 

 muscles posterior to the limb. However, the organization is labile, because 

 these two regions can be interchanged in early development and can be 

 substituted one for the other. This substitution has been effected by the 

 following type of operation. Figure 91 shows a spinal cord from segments 1 

 through 8. Segments 3, 4, and 5 were removed, and from another embryo 



