66 HOW ANIMALS DEVELOP 



They did not self-differentiate and therefore cannot 

 have been determined when the experiment was 

 made: their fates must have been determined by 

 something in their new situations. When the experi- 

 ment was made in the late gastrula stage, the result 

 was the exact opposite. The white piece of pre- 

 sumptive skin developed as skin although it was lying 

 in the middle of the taeniatus brain, and the trans- 

 planted piece o^ taeniatus presumptive brain developed 

 into a little patch of brain lying in the middle of the 

 cristatus skin. The determiner, then, must have 

 finished its action by the late gastrula stage, so that 

 the parts of the egg are by that time determined and 

 must self-differentiate. This fixed the period during 

 which the determiner works. 



Spemann then began to examine the various parts 

 of the gastrula, trying to locate the determiner. He 

 got his first hint of where to look from the following 

 experiment. He took a young taeniatus gastrula, cut 

 it in half with a horizontal cut, and then turned 

 round the top half through half a turn and replaced 

 it in its new position. A glance at Vogt's map of the 

 newt gastrula (Fig. 1 1 ) will show what he had done ; 

 he had exchanged the presumptive skin and neural 

 plate areas. However, the gastrula healed up again 

 and went on developing. The neural plate appeared 

 in front of the blastopore as usual ; in fact, in its 

 normal position as regards the bottom half of the 

 egg. But it was in an abnormal position as regards the 

 top half and must have been made out of presumptive 

 skin. This shows that the determiner must be in the 



