82 ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 



of the main asymmetry-gradient. If the result is merely to flatten 

 this gradient, animals of normal asymmetry will still result. But if 

 the effect of the cut is strong enough to reverse the existing gradient, 

 animals of reversed asymmetry will arise. This applies both to 

 Echinoderms and to Amphibia: in addition, in Echinoderms the 

 almost complete balancing of the two lateral halves of the gradient 

 will give rise to bilaterally symmetrical forms (with both left and 

 right hydrocoels), whereas this result is impossible in Amphibia, 

 where the normal and reversed asymmetries are mutually exclusive 

 alternatives. 



The main points of this chapter may be briefly summarised as 

 follows. In amphibian development, polarity or axiation and bi- 

 lateral symmetry are both established as the result of agencies 

 external to the egg. In both cases, an important effect of these 

 agencies is the production of activity-gradients extending through 

 the whole egg. In the production of bilateral asymmetry, an 

 activity-gradient is also involved. At the moment it is not possible 

 to state what is the originating cause of this asymmetry-gradient ; 

 we do know, however, that its establishment is in some way de- 

 pendent upon the establishment of the dorso-ventral gradient 

 which determines bilateral symmetry. Both these latter gradients 

 appear to become progressively accentuated during the period of 

 cleavage. 



