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INDUCTION AND ORGANISATION 



into the foreign environment of the neural plate, will still 

 produce epidermis (Spemann, 1918). Also in the case of ex- 

 plantation, neural plate material will differentiate into nervous 

 tissue, and skin ectoderm (in the presence of mesenchyme cells) 

 into epidermis (Holtfreter, 1931). Evidently, the two regions 

 of the ectoderm have now acquired different differentiation 

 potencies. At the same time, the range of possible different- 

 iations has been cut down. After transplantation into a foreign 



Fig, 39. Heteroplastic transplantation of presumptive ectoderm at 

 an early gastrula stage, (a) gastrula of Triton taeniatus with graft 

 from T. cristatus; (b) gastrula of T. cristatus with taeniatus graft; 



(c) the taeniatus embryo of fig. (a), at a later stage; the graft 

 (light pigmentation) occupies the front part of the neural plate; 



(d) cross section of this taeniatus embryo; part of the brain wall 

 (between the crosses) has developed from the light cristatus graft, 

 which has produced brain tissue in accordance with its new environ- 

 ment. After Spemann. 



