

The presence of the organiser region is essential for development, a, A newt's egg 

 constricted into two in the transverse plane, thus separating dorsal and ventral 

 halves, b, The result of isolation of a dorsal half (containing the organiser 

 region) : a perfect embryo, c, The result of isolating a ventral half (lacking the 

 organiser region) : a blastula-like ball of cells which develops no further, d, A 

 newt's egg constricted in the plane of symmetry, thus separating lateral halves, 

 each of which contains a portion of the organiser region, and, e, develops into a 

 perfect embryo. (From Spemann, Naturzviss. iv, 1924.) 



