342 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



primary field-system: in the first case presumptive trunk regions 

 actually become head, and vice versa in the second case (figs. 158, 

 160). 



When the temperature-gradient is applied after mid-gastrula- 

 tion, antagonistic gradients often produce neural folds which are 

 much bulkier than normal, while the opposite effect is produced 

 with adjuvant gradients.^ This is also true of the mesoderm. As 

 Gilchrist suggests, this apparently paradoxical effect is presumably 



mm $■■■■■ '.-v. 





Fig. 161 



Three stages in the development of an Amhlystoma embryo treated from the 

 4-cell stage for 3 days after being symmetrically marked with vital stains, the 

 whole right half was inhibited by being subjected to abnormally low temperature. 

 A, On removal from treatment, yolk-plug stage; the normal side shows an 

 incipient neural fold. B, Later; the left neural fold is well developed, the right 

 has still not appeared. C, The right neural fold has arisen and has united with the 

 left; it has, however, been formed out of material to the left of the original mid- 

 dorsal line. (After Vogt, from Gilchrist, Quart. Rev. Biol, iv, 1929.) 



due to the fact that the neural plate is determined by the ingrowing 

 organiser region, whose high point is vegetative, so that high 

 temperature at the animal pole is really antagonistic to the processes 

 leading to neural plate formation (fig. 159). 



In a series of experiments in which lateral temperature-gradients 

 were applied to Urodele blastulae,^ the plane of bilateral symmetry 

 of the Q^g and embryo was actually shifted towards the warmed 

 side. It appears that this is in the main due to alterations of 

 growth of the invaginated organiser; however, since no experi- 

 ments seem to have been performed in which the application of 



^ Gilchrist, 1929; Dean, Shaw and Tazelaar, 1928, text-figs. 6 and 7. 

 ^ Gilchrist, 1928; Vogt, 1928 b, 1932. 



