GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 359 



has been reversed. Such a disc, however, develops into a Hmb with 

 right-hand asymmetry on the right side of the body, akhough it 

 originally came from the left side, and it is the proper way up : the 

 palmar surface of the hand is turned down.^ It has failed to preserve 

 its prospective dorso-ventral polarity, and has acquired a new one 

 in conformity with its new surroundings (fig. 173). 



These experiments show that the antero-posterior axis of the 

 limb-disc was irreversibly fixed before the time of the operation. 

 The polarity thus imposed on the limb-disc determines where a 

 preaxial border (that marked by the radius and first digit) will be. 

 But the dorso-ventral axis is not yet fixed, and the determination as 

 to which side will be the palm and which the back of the hand de- 

 pends on the orientation of the disc with regard to its host. In the 

 antero-posterior axis of the limb-disc it is easy to recognise the 

 primary axis of polarity of the embryo. The main axial gradient of 

 the egg persists, and permeates the limb-disc. The dorso-ventral 

 gradient of the embryo, however, appears to be less powerful or to 

 become active only at a much later stage. 



As regards the medio-lateral axis, it is found that a limb-disc will 

 always develop outwards, away from the body, whether it was 

 grafted the proper or the wrong way out ("medio-medial", or 

 "medio-lateral"), and this shows that the medio-lateral polarity 

 is not fixed in the limb-disc stage. ^ 



^ Harrison, 1921 a; Ruud, 1926. 



- A further point of interest in connexion with the grafts of Hmb-discs is that, 

 at these early stages, it is not "right-handedness" or "left-handedness" that is 

 determined at all. This is made quite clear from the fact that a left limb-disc can 

 be made to differentiate into a right-handed limb on the left side of the body by 

 reversing the antero-posterior axis. (Either, " homopleural, antero-posterior, 

 dorso-ventral, medio-medial " ; or "homopleural, antero-posterior, dorso-dorsal, 

 medio-lateral".) The geometrical configuration of right- or left-handedness is 

 the result of the determination of three axes. One of these, the antero-posterior, 

 is already determined at the stage operated upon. The second axis, the dorso- 

 ventral, is determined later, so that grafts of limb-buds of a more advanced stage 

 of development show a determination not only of the preaxial border, but also 

 of the palmar surface (Brandt, 1924). The third axis, the medio-lateral, seems 

 throughout life to be dependent on the orientation of the limb-rudiment relative 

 to the whole organism, and never to be irrevocably determined. In an adult 

 newt, the left leg may be cut off, and planted into the dorsal side of the animal 

 in such a way that the end which was originally proximal now points outwards. 

 Regeneration takes place from this end, and a bud is formed which proceeds to 

 differentiate into a right leg. The preaxial border and the palmar surface being 

 determined as a result of the original antero-posterior and dorso-ventral axes, a 



