68 The Development of the Vertebrate Embryo 



5. Enzymic changes during development. What enzymic changes 

 accompany the transformation of unspecialized cells into an organized 

 tissue or organ, i.e., which enzymes appear, which disappear, which in- 

 crease or decrease in concentration, what are the control mechanisms? 

 The cataloging of enzymes and enzyme concentrations is now a routine 

 procedure and can be accomplished with ease and precision. 



6. The appearance of tissue-specific substances during embryogen- 

 esis. For example, we can follow the development of the heart in the early 

 embryo by looking for the presence of heart-muscle myosin, a contractile 

 protein unique to this muscle tissue. How to distinguish between heart 

 myosin and the myosins of other muscles? We can purify the heart myosin 

 and inject it into rabbits. Because chick heart myosin is a foreign sub- 

 stance to the rabbit, the animal can make antibodies that will react 

 specifically with chick heart myosin. Thus the rabbit serum containing 

 these antibodies can be employed as a specific indicator of the contractile 

 protein, and we can answer questions such as at what time the myosin 

 appears and from where, how much, and in what cells, at a stage long 

 before the actual heart is evident as a discrete organ. 



You will notice that the above categories are heavily punctuated by 

 question marks. This is as it should be, for chemical embryology is in its 

 infancy and not nearly at the level of sophistication that one would like. 

 Here, perhaps more than any other, is an area open to fruitful investiga- 

 tion by physics- and chemistry-minded young students. 



