142 INDUCTION AND ORGANISATION 



chyme, and the girdle skeleton with its muscles has developed 

 in the flank. In urodeles, the fore-limbs are formed at an 

 early stage, whereas the hind-limbs develop much later. In 

 anurans, on the other hand, both pairs are formed at about 

 the same time. Most of the experiments have been made on the 

 fore-limbs of urodeles. 



In this group, the limb-forming potency can be demonstrated 

 as early as the gastrula stage. If grafted into an abnormal 

 place, the material concerned (ectoderm and mesoderm to- 

 gether) will still develop into a limb (Detwiler, 1929-33). Here, 

 too, the potency initially extends over a larger area, its in- 

 tensity decreasing in all directions from a central maximum 

 (Harrison, 1915-18). Probably the potency is originally located 

 exclusively in the mesoderm. At somewhat later stages, how- 

 ever, when the limb-bud has become visible, the ectoderm 

 covering the bud has also acquired this potency. If at this 

 stage this ectoderm alone is grafted into another place, it will 

 produce a limb in co-operation with the local flank mesenchyme 

 (Filatow, 1930). Rotmann (1931-33) has effected separate 

 heteroplastic transplantations of limb mesoderm alone, and 

 limb ectoderm alone, between Triton taeniatus and T. cristatus. 

 His results prove that the mesoderm has the main influence 

 on the limb's specific form and size. It is not until later larval 

 stages that a slight influence of the ectoderm on the size of 

 the limb and the shape of the digits becomes manifest. 



At first, the limb rudiment has all the properties of an 

 organisation-field: transplanted parts of the primordium can 

 produce a complete, harmoniously built limb, so that one 

 primordium can give rise to as many as four limbs. On the 

 other hand, two fused primordia can produce a single harmo- 

 nious limb. All the cells of the rudiment, therefore, are still 

 able to produce any part of the limb. The field has not yet 

 become tied to certain cells, but it can be displaced, can divide 

 into a number of equivalent fields, or can fuse with another 

 field into one unity. The field can be ''transposed" with regard 

 to the material. 



Harrison (1915-25) carried out a large number of experi- 

 ments on the symmetry relationships of the limbs. Left and 



