360 GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



A State of affairs which presents many similarities with the deter- 

 mination of the axes of the hmb is found in connexion with the 

 development of the ear (see p. 233). The auditory vesicle in Am- 

 phibia arises as a sac formed from the epidermis on each side of the 

 neural tube, behind the eye. When it is first determined in normal 

 development, the ear-rudiment is a field the constituent parts of 

 which are not yet fixed ^ and it is therefore capable of regulation. 

 But the field is polarised with reference to the main axis of the em- 

 bryo, so that if the ear-rudiment is grafted in such a way that the 

 antero-posterior axis is reversed, it develops with reversed asym- 

 metry.2 Comparable grafting experiments have shown that in the 

 rudiments of the external gills and of the heart ^ of Amphibia,^ and 

 of the operculum in Anura,^ the antero-posterior axis is already deter- 

 mined at a stage when the rudiment is still in the condition of a 

 field, capable of regulation after losing a portion of itself. 



Another fact of morphogenesis which appears to depend upon the 

 persistence of the main gradient-system of the organism is the 

 phenomenon of serial heteromorphosis. As is well known, after 

 amputation of an appendage, certain Arthropods may regenerate one 

 of another type, e.g. an antenna in place of an eye in Palaemon, or a 

 leg in place of an antenna in various Orthopteran Insects. Natural 

 examples of this have also been found in various groups. The ab- 

 normally located appendage is, in almost all cases, one belonging 

 properly to a more posterior region of the body.^ This could be ac- 

 counted for if it is assumed that the original morphogenetic gradient 

 persists throughout life, but becomes flattened during later develop- 

 ment, so that anterior structures now come to correspond to a lower 

 level of the gradient than they formerly did during early develop- 

 ment. Since it is known that cold and depressant chemical agents 

 will flatten a physiological gradient (p. 337), it is by no means un- 

 reasonable to assume that increasing age will have the same effect. 



reversal of the medio-lateral axis in the regeneration- bud results in a reversal of 

 the asymmetry of the limb regenerated (Milojewic and Grbic, 1925). See also 

 Harrison, 1925 a. 



1 Kaan, 1926. - Tokura, 1925. 



3 Stohr, 1925; Copenhaver, 1926. ^ Harrison, 1921 b. 



^ Ekman, 19 13. ^ Przibram, 193 1 b. 



