222 PRINCIPLES OF EMBRYOLOGY 



fields into embryology, and who contributed greatly to the early experi- 

 mental work on vertebrate regeneration (see Weiss 1930, 1939). As we 

 shall see however, the field which is operative in ampliibian regeneration 

 is strictly an individuation field concerned with the buildmg up of a com- 

 plete unit, and shows no sign of activity outside the Hmits of this unit. 

 There is therefore nothing which corresponds to the dominance ot the 

 old hydranth in the regeneration of hydroids. . , 1 



As far as regeneration is concerned the urodele body can be considered 

 to be made up of a series of organ-districts. If a complete district is re- 

 moved, It caimot be regenerated. If, however, any part of the district is 

 ehminated regeneration wiU restore what is missing, unless indeed tor 

 some reason the blastema is smaller than normal, when a deficient organ 

 may be produced. The limit of the organ district of the tail, for mstance, 

 is the last sacral vertebra. If the tail is amputated anterior to this, no re- 

 generation occurs, whereas if the cut is made anywhere further posteriorly, 

 a complete tail is formed. Regeneration occurs strictly withm each organ 

 district and is uninfluenced by the position of that district withm the body 

 as a whole. Thus if a limb is transplanted to the middle of the back and 

 then amputated, the stump nevertheless regenerates a hmb. The character 

 of the regenerate, m fact, depends on the tissue in the immediate neigh- 

 bourhood of the wound. If a hindUmb is amputated, a forehmb trans- 

 planted to the stump, and then the forehmb agam amputated so as to 

 leave only a small section of it, the regenerate will still be a forehmb and 

 show no influence of the hindhmb stump, which is further away from the 



wound surface. 1 • 1 



Amphibian regeneration exhibits several pecuhar polarity phenomena. 

 In the first place it should be noted that the regenerate always produces 

 the parts distal to the wound surface and apparently never those proximal 

 to it Thus if a deep V-shaped cut is made into a hmb, both faces of the 

 wound may produce a regenerate and both of these will develop into 

 the regions of the hmb which should he distal to the cut. Agam, it 

 a segment of a hmb, say the region near the knee joint, is isolated by two 

 cuts and then inserted into the body-wall in such a way that the Proximal 

 surface as well as the distal can regenerate, it will be found that the 

 proxknal surface does not form a new femur and upper part of the hmb 

 but that both surfaces form the distal extremity. J. Needham (1942 

 accepted some earlier data which suggested that in such cases the proximal 

 surface regenerated only so much of the lower limb as had origmally been 

 isolated. This would have been a very pecuhar situation, since it would 

 have meant that the mdividuation field had been permanently altered by 

 the isolation. It appears, however, from more recent work of Monroy 



