358 



GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



disc is grafted on to the right side of the body, the proper way up 

 and the proper way out (" heteropleural, antero-posterior, dorso- 

 dorsal"), only the antero-posterior axis has been interfered with 

 and reversed. In such case, the disc develops into a limb with a 

 left-hand asymmetry on the right side of the body, with elbow 



Fig. 173 



Diagram illustrating experiments on the symmetry of limbs. The circles 

 represent the limb-buds as grafted on to the right side of the body. The letters R 

 and L in the centre of the circles indicate the side of origin of the bud (right or 

 left). The letters A, P, D, V inside the circle indicate the antero-posterior and 

 dorso-ventral axes of the grafted bud, these letters outside the circle refer to the 

 same axes of the body of the organism. The limb which develops is shown with a 

 thick outline. The position of a reduplicated limb (should one develop; see 

 footnote, p. 224) is indicated by the fine outline; the dotted line refers to the 

 form which the limb would have taken if the dorso-ventral axis of the bud had 

 been fixed at the time of grafting. Only medio-medial combinations are shown. 

 (After Harrison, from de Beer, Biol. Rev. 11, 1927.) 



pointing forwards and hand pointing backwards : it has preserved 

 its prospective antero-posterior polarity, in spite of the reversal of 

 this relative to the body as a whole. But if a left limb-disc is grafted 

 on to the right side of the body, the proper way out but upside 

 down (" heteropleural, antero-anterior, dorso-ventral"), the antero- 

 posterior axis has been respected and only the dorso-ventral axis 



