362 GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



for the existence and persistence of this gradient is to be found in 

 the asymmetry of the reproductive system. It will be remembered 

 that the asymmetry-gradient gives rise to a general preponderance 

 of the left side. This is apparent also in the gonads. When only 

 one gonad becomes functional, as in female birds and monotremes, 

 it is the left. Further, in normal development, e.g. of frogs, the left 

 gonad in both sexes is usually the larger.^ The left testis is larger 

 than the right in many species of birds.^ With this may be asso- 

 ciated the fact that in intersexual mammals the left gonad tends to 

 be more female, the right gonad more male.^ 



We may also mention the interesting fact that in genetic Poly- 

 dactyly in birds, when, as sometimes occurs, the extra digit is formed 

 only on one leg, this leg is usually the left.* 



§4 



The persistence of regional fields to later stages has been demon- 

 strated in adult Vertebrates capable of regeneration, by experi- 

 ments in which nerves are deflected from their normal course and 

 left to end in various regions close under the skin. In the newt, for 

 instance, if a brachial nerve is diverted from its normal course and 

 led away so as to end freely within a certain area surrounding the 

 arm, the growth of a supernumerary arm is initiated : if it is led into 

 an area close to the dorsal fin (or crest), a supernumerary piece of 

 crest is induced. Similarly, a sciatic nerve deflected into the region 

 of the arm or of the tail causes an extra arm or an extra tail to arise. 

 In the lizard the area at the base of the tail can be stimulated to 

 form a supernumerary tail by the sciatic nerve. ^ 



Thus round the arm, in the newt, there exists an area which re- 

 tains the potency of arm-production even in adult life. This area 

 has been appropriately called the arm-field.^ Similar fields exist 

 for the tail, leg, dorsal crest, etc. Other evidence, confirming this, 

 is provided by the experiments recorded in Chap, viii (p. 271), 

 in which undetermined regeneration-buds of newts grafted into 

 abnormal situations produced organs characteristic of their new 

 situations, and not the type of organ by which they had been budded 



1 Cheng, 1932. - Friedmann, 1927. 



3 Baker, 1926. ^ Bond, 1920, 1926. 



^ Guyenot, 1928. '^ Guyenot and Ponse, 1930. 



