458 PRINCIPLES OF EMBRYOLOGY 



embryo. (Extensive studies on regional specificity during gastrula stages 

 have been made by Okada and a group of Japanese workers (see Hama 

 1950), but seem to suffer from lack of adequate statistical evaluation.) 

 One is forced to admit that the regional character of the various parts of 

 the mesoderm is only imprecisely determined during the process of 

 gastrulation and is still to some extent labile. 



This is indeed what one might expect from other types of experimental 

 evidence. Thus, as pointed out earUer (p. 190) Yamada (1940) has shov^oi 

 that even in the neurula the 'level' of the mesoderm on the dorso-ventral 

 axis is not yet fmally fixed; for instance, presumptive lateral plate can be 

 forced to develop into somitic muscle if notochord is brought into its 

 neighbourhood. A similar flexibility occurs along the anterior-posterior 

 axis. Waddington and Yao (1950) showed that if presumptive anterior or 

 posterior portions of the organiser are exchanged in young newt gastrulae, 

 completely normal individuals may be produced, which must involve 

 an alteration in the anterior-posterior specificity of the exchanged region. 

 (In similar experiments with the rapidly developing gastrula of the 

 anuran Discoglossus [Waddington 1941] the morphogenetic tendencies 

 of a graft were so strong that they prevented its incorporation by the host 

 and no redetermination of the regional character could be proved in that 

 case.) Even at the end of gastrulation considerable flexibility still persists 

 along the anterior-posterior axis, since a fairly normal embryo (with 

 over-thick mesoderm) may develop if an extra archenteron roof is added 

 with reversed orientation between the normal archenteron roof and the 

 presumptive neural plate (Fig. 20.23). 



Figure 20.23 



Some experiments on the regional properties of the organiser. On the left, 

 the presumptive neural plate of a late gastrula folded back, a second archen- 

 teron roof laid with reversed orientation over the original roof, and the 

 ectoderm returned to place. On the right, reversal of the dorsal lip region. 

 (From Waddington and Yao 1950.) 



