2/6 Ross Granville Harrison 



without their spinal cords. And yet when the hmb of such an 

 individual is transplanted to a normal larva it acquires a complete 

 and normally arranged system of peripheral nerves just as a limb 

 taken from a normal larva does. In other words, lack of func- 

 tional activity, consequent upon the absence of nerve centers dur- 

 ing a protracted and important period of development, does not in 

 the least interfere with the later normal development of the nerves 

 as soon as new nerve centers are supplied. These nerves, there- 

 fore, must be regarded as the product of the nerve centers alone. 

 This answer to the question being firmly established as correct, 

 the cardinal importance of Braus' fundamental expernnent becomes 

 apparent, for, contrary to the conclusion drawn by its author, it 

 shows that the structures contained within the limb must have a 

 very important directive action upon the developing nerve fibers, 

 in that they determme their mode of distribution. The manner 

 of branching cannot possibly be predetermined in the ingrowing 

 nerves themselves, because m the normal body these same nerves 

 have an entirely different distribution. Let us picture to ourselves 

 what probably takes place after a limb is transplanted. It is put 

 into a region well supplied with nerves. The wound made tor the 

 reception of the bud involves without fail injuries to the nerves of 

 the region. This stimulates the fibers to grow and in so doing some 

 of them will come into contact with the cells of the transplanted 

 bud, which at that time consists of a blastema of mesenchyme 

 cells covered by epidermis but not visibly differentiated. As the 

 bud grow§ into a leg and the blastema differentiates, the nerve 

 fibers become arranged and segregated according as they are at- 

 tached to the organs within the limb. In the limbs in normal posi- 

 tion the development of the nerves must go on in the same way. 

 Here too the nerve fibers reach the bud when it is still without 

 visible differentiation. Contact with the cells contained within it 

 being made at that time, the peripheral branches of the nerves 

 are determined as the constituent parts of the limbs are segre- 

 gated. The fact that any nerve in whose way a limb is planted is 

 capable of giving rise to intrinsic nerves having perfectly normal 

 arrangement, shows that the nerves themselves must be guided in 

 the formation of their terminal ramifications. 



