organisers: inducers of differentiation 147 



regional differences in different portions of it. It will be realised 

 that that portion of the organiser area which is the first to become 

 invaginated at the rim of the dorsal lip of the blastopore will reach 

 furthest forward and come to underlie the head, while that portion 

 which becomes invaginated later will come to underlie the trunk. 

 It has in point of fact been found that these two portions of 

 the organiser show a regional difference as regards their power of 

 induction. For instance, "head-organiser" (invaginated early), 

 grafted at head level in the host, will form the cephalic axial 

 structures (brain, eyes, ears) as might be expected, and the 

 secondary embryo so formed may lack the trunk region. On the 

 other hand, "trunk-organiser" (invaginated late), grafted at trunk 

 level in the host, will form the axial structures characteristic of the 

 trunk, and such secondary embryos will lack brain and eyes, and 

 in many cases ears as well (figs. 67, 68; see also Appendix).^ 



Similarly, with regard to homoiogenetic induction by the neural 

 tube, it is found that anterior portions induce the formation of 

 anterior cephalic structures (e.g. eye), middle portions induce 

 posterior cephalic structures (e.g. ear), while posterior portions 

 induce structures characteristic of the trunk and tail.'^ 



These facts make it clear that there exists a regional differentia- 

 tion within the organiser area itself. The result of induction, how- 

 ever, is also dependent on the level along the main axis of the host 

 of the tissues upon which the organiser exerts its action. This is 

 shown by the following experiments. Head-organiser grafted at 

 trunk level in the host will induce the somewhat imperfect forma- 

 tion of cephalic axial structures, including brain, eyes, and ears. 

 On the other hand, trunk- organiser grafted at head level in the host 

 can also produce these cephalic structures, but eyes will only be 

 formed if the anterior end of the neural tube of the secondary em- 

 bryo reaches forward as far as the level of the eyes of the primary 

 embryo.^ 



Thus, as noted above (p. 140), the host-tissues are not without 

 influence on the formation of the secondary embryo. As a general 

 rule, it is found that the secondary embryo is arranged with its long 

 axis roughly parallel with that of the primary embryo, or, in other 



^ Spemann, 1927, 1931; Bautzmann, 1929 A, 

 ^ Mangold, 1929 b; 1932. 



