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EMBRYOLOGY 



Fig. 33. A living salamander neurula 

 just after the primitive nervous system, the 

 neural plate, has formed. The neural plate is 

 folding to form a neural tube, and the large, 

 broad brain region can be distinguished from 

 the narrow spinal-cord region. 



The demonstration of a single potency is very simple and is shown in 

 Figure 34. The same host is used as in the previous experiment, but in this 

 instance the donor gastrula was allowed to develop into a neurula with a 

 well-formed neural plate. Part of the anterior neural plate is cut out and 

 transplanted into the flank region of an older embryo. Under these condi- 

 tions this piece of tissue develops and forms a perfect eye in the flank region 

 — and nothing more. No somites or notochord develop as in the previous 

 experiment, in which the presumptive eye of the gastrula was transplanted. 

 Between the stage of the early gastrula and the stage at which the neural 

 plate is present this piece of tissue has become restricted in its potencies 

 and can form eye, and eye only. The rest of the nervous system behaves in 

 a similar fashion, with the result that any part of the brain of the neurula 

 will form only brain when transplanted. It is apparent, therefore, that 

 during gastrulation the entire nervous system becomes limited in its potencies 

 and can no longer develop into other structures. 



The induction of the primitive 

 nervous system 



With this analysis of the potencies of the ectoderm as a background we 

 can go on to ask what it is that selects, out of all the possible modes of 

 development of the presumptive neural plate, the potency for nervous sys- 



