organisers: inducers of differentiation 



SI 



formation of neural folds in these strange positions, with the result 

 that two perfect heads and anterior trunk regions are formed. 

 Further, it may be noted that it is impossible by the method of 

 partially constricting gastrulae to obtain duplicitas posterior, or 



■'^T^<. 



Fig- 75 

 Duplicitas cruciata, obtained by grafting together two gastrula-halves (see fig. 73) ; 

 the heads and anterior regions of the trunk have a plane of symmetry which is at 

 right angles to that of the posterior regions of the trunk ; the former are seen in 

 ventral, the latter in side view. One of the heads has a cyclopia eye. (From 

 Wessel, Arch. Entwmech. cvii, 1926.) 



doubling of the hind end. This is obviously because the constric- 

 tion forces the anterior but not the posterior part of the primitive 

 gut-roof to fork (fig. 169). 



On the other hand, both anterior and posterior doubling can be 

 obtained by grafting together halves of gastrulae in such a way that 

 their original planes of symmetry (and therefore, directions of 



