3l6 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



§ 2. The domina7it regioji in ontogeny 

 (iii) Independence of the dominant region 



Instances of this are difficult to obtain in ontogeny. The egg cannot 

 regenerate new tissue Uke a Planarian worm, and we can therefore 

 only compare the morphogenetic processes occurring in it to those 

 occurring by morphallaxis in the regeneration of, for example, a 

 piece of Tuhidaria stem. However, the gradient-system of the egg 



Fig. 148 

 Differentiation of notochord {nc.) and mesoderm {ms. muscle) from animal pole 

 material (presumptive epidermis and/or brain) of Triton interplanted in the eye- 

 socket of a larva of Triton taeniatus. (From Bautzmann, Naturwiss. xvii, 1929.) 



is almost always more specialised than that of a hydroid stem, being 

 in many cases partly or wholly determined as regards different 

 levels along its main axis. It is also more limited in size, and there- 

 fore its gradient is presumably steeper. 



However, when both animal and vegetative portions of the ^gg 

 can reorganise themselves to form perfect wholes, as in some 

 Coelenterates (p. 97), equatorial portions of the egg, originally in 



