158 organisers: inducers of differentiation 



organiser-invagination) either diverge or converge anteriorly.^ 

 In the former case, the compound embryo will have a Y-shaped 

 primitive gut-roof with the divergence anterior, and will develop 

 double heads; in the latter case the divergence will be posterior, 

 and there will be double hind ends. 



Perhaps the most remarkable cases of teratological development 

 induced experimentally are those producing crossed doubling 



Herz 









Med. 



Herz 

 Fig. 76 

 Transverse section through a duplicitas cruciata embryo of Triton, such as that 

 shown in fig. 75. The hearts {Herz) are formed partly from each embryonic 

 rudiment, and are therefore situated laterally. Med., neural tube. (From Wessel, 

 Arch. Entwmech. cvii, 1926.) 



(duplicitas cruciata). These result from the grafting together of two 

 gastrula halves each containing the dorsal lip, in such a way that the 

 directions of organiser-invagination are directly opposed to one 

 another. Invagination takes place in each half, and the primitive 

 gut-roofs meet one another, head on. Being unable to make any 

 further progress forwards, they move to each side. The result is 

 that the primitive gut-roofs together form a cross, two (opposite) 

 branches of which are formed each from one of the two invagina- 

 tions, and the other two branches are composite, half of each being 



^ Spemann, 1916, 1918; Koether, 1927. 



