THE "organization CENTRE*' 67 



bottom half; Spemann suggested that it is actually 

 located near the blastopore. 



A few years later Spemann and his pupil, Hilde 

 Mangold, tested this suggestion. They removed the 

 blastopore region from a young cristatus gastrula and 

 grafted it into the belly region of a taeniatus gastrula. 

 The taeniatus gastrula, which thus had two blasto- 

 pores, developed two neural plates, and two 

 complete sets of embryonic organs (Fig. 17). This 

 might have been due simply to a self-differentiation 

 of the grafted blastopore, but Spemann and Mangold 

 showed that this was not a complete explanation. 

 Owing to the differences in colour between the cells 

 of the taeniatus host and the cristatus graft, they could 

 tell how the secondary embryo was built up. 

 Examination showed that the grafted blastopore had 

 developed chiefly into mesoderm, which was its 

 presumptive fate, while the neural part of the 

 secondary embryo had been formed from the tissues 

 of the host. The mesodermal part therefore had 

 arisen by self-differentiation of the grafted blastopore, 

 but the neural part had not. It had been induced out 

 of host tissue which would ordinarily have formed 

 skin. The implanted blastopore had therefore 

 determined the fate of this presumptive skin, causing 

 it to become actual neural plate. This experiment 

 definitely locates the determiner at the blastopore. 

 It was later showTi that the whole ring of mesoderm 

 which lies in front of the blastopore in Vogt's map 

 is capable of determining, though this capabiHty is 

 strongest just near the blastopore and becomes 



