8 • THE EMBRYO: DIFFERENTIATION 

 WITHIN FIELDS 



W 



V Veh 



e have now reached a point in our analysis of the developing egg 

 where the major structures of the embryo are determined and will self- 

 differentiate when they are transplanted to a foreign environment. Does this 

 mean that developmental processes are over and that all the details of the 

 structure and function of the adult are now determined ? Is a structure such as 

 the limb blocked out in detailed mosaic at this early stage? Recall that we 

 asked the same questions regarding the organization of the egg at the begin- 

 ning of development. In the case of the egg two possibilities presented them- 

 selves. Either the structures of the adult were preformed in the egg or there 

 was some type of labile organization. And in the sea urchin egg experimenta- 

 tion demonstrated a labile organization which could be modified by chemical 

 treatment. 



The polarity of the limb disk 



What is the situation with regard to the organization within the major 

 structures of the body, such as limb, eye, heart, ear, and others? For an 

 answer we resort to the same types of procedure that we used with the egg. 



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