84 Cell interactions during Growth and Morphogenesis 



somites. From this point on, many contiguous tissues provide each other 

 with inductive signals that trigger the formation of additional organs. In 

 general, the proof that such inductions operate has fallen into two cate- 

 gories: investigators have shown (a) that if tissue A is removed or physi- 

 cally separated from tissue B, tissue B will not construct a specific organ 

 as it usually does; and (b) that if tissue A is moved to a neighboring spot 

 occupied by cells closely allied to B, the organ in question will form at the 

 new site occupied by tissue A and not at the old. 



Figure 40 is a schematic and partial summary of the manifold induc- 

 tive relationships uncovered in this manner. In some cases, these induc- 

 tions have, by and large, proved to be much more specific than that 

 supplied by the embryonic organizer. However, little of the rigorous 

 biochemistry needed to shed light on them has been performed so far. 



Fig. 40. Inductive relationships between different parts of the developing 

 embryo (after Holtfreter). Arrows point from the inducing tissues to the 

 induced organ that ultimately appears. Bear in mind that this array of in- 

 ductions is far from complete. 



Anterio 

 mesoder 



Forebroin 



I No 



Somites 



Lateral 

 mesoderi 



Central 

 nerve chord 



Digestive tract 



Neural crest cells 



Skin 



Dorsal fin 



