144 



Head-Organiser ^n- Head Level 



Heeid-Organiser w Trunk Level 



R. sec. 

 a.v. sec. eye I. 



Trunk-Organiser in Head Level 



Trunk-Organiser ztt Trunk Level 



Fig. 67 



Diagram showing the results of experiments testing the inductive capacities of 

 head-organiser (invaginated early: the anterior region of the primitive gut-roof) 

 and of trunk-organiser (invaginated late: the posterior region of the gut-roof), 

 and the reacting capacities of the host-tissues at head level and trunk level (see 

 also fig. 68). Head-organiser at head level forms only the head of a secondary 

 embryo with eyes and ear- vesicles ; head-organiser at trunk level may form a 

 complete secondary embryo, and the cephalic structures may arise at a level 

 considerably behind those of the primary embryo ; trunk-organiser at head level 

 may form a complete secondary embryo with cephalic structures at levels more 

 or less corresponding to those of the primary embryo ; trunk-organiser at trunk 

 level produces the trunk of a secondary embryo ; ear- vesicles are formed if the 

 secondary embryo reaches to the level of those of the primary embryo. Head- 

 organiser can thus form a head in both head and trunk levels, but trunk-organiser 

 can only form a head in head level ; the reaction to trunk-organiser of the host- 

 tissues at head level is to form a head, and at trunk level to form a trunk. 

 Pr.o.v. eye; Pr.a.v. ear, or primary embryo; sec.o.v. eye; sec.a.v. ear (left, L. or 

 right, i?.); sec.cycl.o.v. cyclopic eye, of secondary embryo. (Original, based on 

 Spemann.) 



