156 organisers: inducers of differentiation 



treatment of monstrosities which conform to the teratological types 

 known as anterior, posterior, and crossed doubhng {duplicitas an- 

 terior, duplicitas posterior, and duplicitas cruciata), is expUcable only 

 in terms of these functions of the organiser. 



^o 







— sek. Med. I 



sek. Med. II 



prim. Schw. 



Fig. 74 

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

 nearly the whole extent of each embryonic rudiment {sek. Med. I, II) is composite 

 and derived partly from each of the half-gastrulae ; only the tips of the tails (prifn. 

 Schw.) are uncrossed, i.e. each formed from one of the half-gastrulae. One of 

 the trunks {sek. Med. II) is less well developed than the other, and ends 

 anteriorly in a knob(*). (From Wessel, Arch. Entwmech. cvii, 1926.) 



If the tgg of a newt is partially constricted in the plane of bi- 

 lateral symmetry during the period of gastrulation, the resulting 

 embryo will show anterior doubling, i.e. it will have two more or 

 less perfectly formed anterior ends joining on to a single posterior 

 end.^ The explanation is that when the primitive gut-roof becomes 

 invaginated, it finds an obstacle in the constriction and has to fork, 

 one portion going forward on .each side of the constriction. The 

 organiser or primitive gut-roof is therefore Y-shaped, and its an- 

 terior prongs underlie tissue which would normally not have given 

 rise to neural folds. But the action of the organiser induces the 



^ Spemann, 1903; Hey, 191 1. 



