INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 463 



perhaps more plausible in Nieuwkoop's case, does not seem by any means 

 necessitated by the data. 



His hypothesis is, however, supported by the recent work of Eyal- 

 Giladi (1954), who claims to have shown that when any part of the meso- 

 derm acts on ectoderm the first result is to confer on the latter the capacity 

 to differentiate into neural crest, but that this stage is fairly rapidly passed 

 through, and in the next period the ectoderm is always induced to form 



y.^ 



Figure 20.26 



Flaps of competent gastrula ectoderm are grafted on to a host embryo in 

 such a way as to become attached to the forebrain, hindbrain or spinal cord. 

 The differentiation of the flaps is indicated by : dotted outline, epidermal ; 

 cross-hatched, mesectodermal ; longitudinal lines, forebrain; crosses, more 

 posterior neural regions. The diagram illustrates Nieuwkoop's interpreta- 

 tion, which supposes that at first the whole of the induced region acquires a 

 tendency to form forebrain, and that in the more posterior flaps this is later 

 transformed towards more spinal development. (From Nieuwkoop 1952.) 



forebrain, while if the inductive action lasts even longer the transforming 

 action will cause the production of more posterior structures. However, 

 the stage when the ectoderm will develop only into forebrain was not 

 noticed by Mangold and von Woellwarth (1950) who isolated pre- 

 sumptive neural tissue from the mid-gastrula of Triton, and its existence 

 can perhaps not yet be taken as certain. 



It will be clear from the last few paragraphs that most recent authors 

 consider that at most very few different substances are involved in the 

 regional determination of the neural plate. Dalcq discusses the question 

 in terms of two, one for the prechordal plate and one for the rest of the 

 chorda-mesoderm. Nieuwkoop considers that there are only two factors, 



