230 



THE MOSAIC STAGE OF DIFFERENTIATION 



develops into an unmistakable arm, but it may have five digits. This 

 is the normal number of digits of the amphibian leg, while the 

 normal arm only has four ^ (fig. 113). In this case, we may suppose 

 that the larger nerve-trunk supplying the leg exerts a trophic effect 

 on the growing rudiment, leading to a condition in which the 

 distal region tends to be meristically divided into five instead of 

 four digits. This is the converse of the results obtained after 



Fig. 113 

 Modification of the arm-bud when grafted into the leg-region. The amphibian 

 arm ends in four fingers (i , 2, 3, 4) ; the leg in five toes (i , 2, 3 , 4, 5). An arm-bud 

 of a white axolotl, grafted into the leg-region (after extirpation of host's leg) of 

 a black axolotl, develops into a typical arm {Tr), except that it possesses five 

 fingers. (From Ruud, Arch. Entzumech. cxviii, 1929-) 



inducing subnormal development in the centres of the mid-brain 

 by extirpating the rudiments of fore-limb or eye : - in these cases 

 the hind-limb was usually malformed and under-developed, and 

 frequently possessed only four or even three toes. Curiously enough, 

 totally denervated hind-limb rudiments, though their growth is 

 reduced, are not malformed, and develop the normal complement 

 of five toes.^ It may be supposed that abnormal conditions in 



1 Ruud, 1929. ^ Diirken, 1925, 1930. ^ Hamburger, 1928. 



