88 



Cellular Structure and Activity 



histochemical results has shown that the 

 tremendous quantities of alkaline phospha- 

 tase which accumulate in the chick intestine 

 at hatching are largely restricted to the 

 presumable functional localization in the 

 striated border (Moog, '50, '51). If this is a 

 general situation — and other cytochemical 

 studies indicate that it is — one may be justi- 

 fied in concluding that differentiating cells 

 control the activity of newly synthesized en- 

 zymes by placing them immediately in the 

 positions in w^hich their activity can be 

 directed and used. 



Thus the middle of the twentieth century 

 finds us with but a rudimentary understand- 

 ing of the field of metabolic regulations dvir- 

 ing development. Plainly we do not have the 

 answers to either of the two aspects of the 

 problem as stated at the beginning of this 

 section. Before we can hope to understand 

 either the way in which the differentiating 

 threads of the total metabolism are woven 

 together into a unified whole, or the way 

 in which morphogenesis emerges from the 

 controlled metabolism, further developments 

 along two lines are necessary. First, our 

 knowledge of the control of embryonic en- 

 zymology can move forward only as fast as 

 our general knowledge of the ultrastructure 

 of protoplasm advances. In this realm \m- 

 doubtedly lie many explanations that we can 

 only hint at now. Second, within the field 

 of embryology itself, more study of isolated 

 tissues is indicated. Although what has been 

 called the actuarial approach to embryonic 

 enzymology has yielded some valuable in- 

 formation on the enzymatic characteristics 

 of differentiating tissue, the validity of such 

 a method is obviously limited. It is with the 

 aid of the methods of histochemistry and 

 cytochemistry that we may expect biochem- 

 ical embryology to make its next steps for- 

 ward. 



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