organisers: inducers of differentiation 



155 



which would otherwise be obscure. As regards the ordinary data of 

 comparative embryology, this property of the organiser makes it 

 possible to understand why there is a correlation between the width 



Fig. 73 

 A, Two dorsal gastrula-halves of Triton grafted together so that the directions of 

 invagination of their blastopores are directly opposed. B, The resulting embryo, 

 showing crossed doubling, or duplicitas cniciata ; each half-gastrula has produced 

 a posterior trunk region with spinal cord, but two heads and brains are formed, at 

 right angles to the axis of the trunks, each formed partly from both half-gastrulae. 

 (Redrawn from Morgan, Experimental Embryology, New York, 1927, after 

 Spemann.) 



of the neural plate and the width of the primitive gut-roof in 

 different groups of Vertebrates:^ the former is dependent on the 

 latter. 



Turning to experimental results, the production by operative 



1 Marx, 1925. 



