278 Ross Granville Harrison 



factorily explaining such variations. ^^ The interpretation of the 

 transplantation experiments just given avoids this difficulty, for 

 it brings out the fact that there are two main determining factors 

 in the development of the innervation of a limb. The first of these 

 is the position and extent of the extremity at the time of origin; this 

 determines the source of the nerve supply. The second factor is 

 the mode of segregation and growth of the individual structures 

 of the limb, which determines the intrinsic distribution of its nerves. 

 The experiments show that these two factors may be varied inde- 

 pendently of one another. Variations in the position and extent 

 of the rudiment of a limb, which may be assumed to occur fre- 

 quently in nature, will, therefore, result in the ingrowth of differ- 

 ent metameric nerves, and still the intrinsic distribution of their 

 branches may remain constant, owing to the circumstance that 

 the factors determining the latter operate in the same way regard- 

 less of the source of the nerves upon which they have to act. Both 

 individual variations and specific differences in the metameric ori- 

 gin of limb-plexuses are naturally explained in this way. 



SUMMARY 



1 Limb buds of tadpoles, when transplanted to various parts 

 of the body of normal individuals, develop normally and acquire 

 usually a complete or partially complete system of peripheral nerves, 

 which have normal arrangement and are connected with the nerves 

 of the host supplying the region in which the limb is implanted. 



2 The whole trunk region of an embryo may be made 

 "nerveless" by cutting out the medullary cord posterior to the ear 

 vesicle, just after closure of the medullary folds. Limb buds taken 

 from such individuals and transplanted to normal larvae behave 

 exactly like the normal limb buds as regards the acquirement of 

 nerves. 



3 Accessory limbs, which frequently develop from transplanted 

 buds by a process of super-regeneration, receive nerves either dir- 

 ectly from the host or from nerve trunks running to the primary 

 transplanted extremity. Sometimes the innervation of the acces- 



^'' Die metamere Umbildung, wie sie sich als Verschiebung zeigt, bleibt damit ein Problem, dessen 

 Lbsung man sich vorlaufig nur mittels der Hypothese nahern kann.'' Gegenbaur, op. cit., p. 613 



