The cell physiology of early development 



by 



C. H. WADDINGTON 



Institute of Animal Genetics, University of Edinburgh 



It is during embryonic development that the fundamental character of the cells 

 composing the organism is formed. The cellular processes occurring at this time 

 must be of a somewhat special and particularly interesting kind. Whereas adult cells 

 synthesize some molecular species essentially similar to those which are already 

 present, during embryonic development and differentiation new types of synthesis 

 must come into operation. I wish in this paper to discuss what sort of picture our 

 present theoretical outlook allows us to form of these processes. It is only in the light 

 of some theoretical scheme that we can decide what are the most important questions 

 to ask, and in what directions we should look for further insight into the fundamental 

 nature of the mechanisms with which we have to deal. 



It will probably be well to start by reminding ourselves of the general type of 

 phenomenon with which we are confronted during differentiation. There are, of 

 course, innumerable examples which we might take, but let us consider the develop- 

 ing tissue of the eye of a vertebrate such as the newt. By the time the fertilized egg 

 has reached the stage where it is ready to gastrulate, the presumptive eye tissue is 

 located in the roof of the blastocoel. The tissue is still capable of developing into a 

 large number of different things. It can form any of the ectodermal derivatives, and 

 it can also be converted into mesoderm and become any mesodermal type of tissue. 

 It has, perhaps, already lost the capacity to form endodermal tissues. By the end of 

 gastrulation the presumptive eye tissue is located in the neural plate. It is no longer 

 capable of developing into any of the derivatives of the main axial mesoderm, or of 

 the non-neural ectoderm. It is, however, beginning to acquire the capacity to develop 

 without further stimulus from outside itself into relatively specific types of neural 

 derivative. Within a fairly short time the range of possible types of tissue into which 

 it can develop becomes more narrowly restricted, while at the same time the re- 

 sponses of the cells to external stimuli are reduced. Recent work (cf. Nieuwkoop and 

 others, 1952) makes it probable that the tissue first acquires the capacity to develop 

 into the various derivatives of the ectomesodermal neural crest, and shortly after- 

 wards that to develop into anterior regions of the brain. At this stage it is still capable 

 of responding to a trunk organiser, which can convert it from forebrain neural tissue 

 into the neural system of the hindbrain or trunk. By the stage of the neural groove, 

 however, the tissue has been rather firmly determined to develop into the eye. It can 

 do so autonomously when isolated from the rest of the embryo and placed in a neutral 

 medium. It has, however, still some flexibility, since a particular group of cells may 

 form either part of the eye stalk, or of the retina, or of the tapetal layers of the eye 

 cup. It is only some time later, after the optic vesicle has folded out from the brain 



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