Experiuieiits in Transplanting Limbs 279 



sory limb is more complete than that of the primary, though more 

 frequently the reverse is the case. 



4. It is possible to keep a nerveless larva alive for a period of a 

 month by grafting it to a normal larva, which acts as a nurse. 

 When a normal extremity bud is transplanted to such a nerveless 

 larva, the nerve twigs contained within the former soon degenerate 

 and no signs of progressive development of the nerves in such cases 

 are to be observed. There is no evidence that an embryonic nerve 

 can continue its development after its connection with the center is 

 severed and prevented from being re-established. Cases which 

 have been reported to the contrary are to be attributed to the pres- 

 ence of anastomoses. 



5 The nerves are not formed in situ in the transplanted limbs 

 but grow into them from the nerves of the host. Experiments 

 which have previously been reported permit of no other conclu- 

 sion and this is strongly reinforced by the experiments with nerve- 

 less limbs. Hensen's theory of primary continuity between nerve 

 center and end organ is untenable, nor does functional activity 

 play any part in the early development of the nerve paths. 



6 Nerves reach the limbs, both natural and transplanted, when 

 the limbs are in the earliest stages of their development and are 

 composed of an undifferentiated blastema of mesenchyme cells. 

 The intrinsic distribution of the nerves is determined by the struc- 

 tures within the limb, most probably at the time when the cells of 

 the blastema segregate into the various definitive structures. This 

 follows necessarily from the fact that any nerve which is led to enter 

 the limb will assume the normal arrangement for that limb. 



7 There are thus two important factors determining the inner- 

 vation of a limb : First, its position and extent at the time of origin ; 

 upon this the source of nerve supply depends. Second, the struc- 

 tures within the limb itself; these fix the mode of distribution of 

 the nerves. 



8 These two factors are entirely independent of one another. 

 Meristic differences in nerve supply of limbs, either between individ- 

 uals or between species, which may exist without affecting the 

 intrinsic distribution of the nerves, are to be regarded simply as 

 due to variations in the original position and extent of the limb 

 rudiments. 



