CHAIRMAN S INTRODUCTION 473 



development of the egg but in the late gastrula or in the neurula stage 

 restrictions occur that have been demonstrated particularly in endodermic 

 nuclei. With respect to stabilization a variety of different conditions must 

 be expected according to the organism under consideration. In vegetalized 

 sea urchin larvae islets of ectoderm may, under certain conditions, arise 

 within the vegetal endoderm demonstrating a late revival of apparentlv 

 extinct pathwavs [22]. Certain forms of regeneration in adult organisms 

 may mean a similar revival.* 



Xo doubt the views established in microbiological research would allow 

 a unitary conception of the early labile embryonic determination and of 

 the later stabilization which still leaves the door open for revival of sup- 

 pressed pathways. To get the firm basis for generalizations of this kind an 

 intensified interaction between morphogenetic, genetic and biochemical 

 studies will be necessary. 



My rapid account on differentiation started from a rather special case. 

 Nevertheless it may have demonstrated some of the principles and prob- 

 lems involved in this function that results in a gradual unfolding of a 

 multitude of structures. The function is based on the structure of the 

 genetic system but without the interactions that arise during the develop- 

 ment the normal pattern of differentiation would not be realized. 



References 



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4. Briggs, R., and King, T. J., /;/ "The Cell", \'()1. I, ed. J. Brachet and A. E. 

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* I am indebted to Dr. F. Jacob, Paris, for stimulating discussions concerning 

 the subject of this paragraph. 



