REGULATION 151 



Consider haemoglobin synthesis in the embryo. The cells of the gastrula 

 do not make haemoglobin ; it is certain that the cells from which the blood 

 islets originate contained the gene which specifies the sequence of amino 

 acids in haemoglobin; synthesis of this protein, however, did not take 

 place to any appreciable extent, it was prevented in some way. Controlling 

 or regulatory processes therefore certainly play a part in the differentiation 

 of protein synthesis. 



Interesting studies were made on the appearance of a few highly 

 specialized or characteristic proteins during differentiation. Thus lens 

 proteins are not detected by a serological precipitation method before the 

 appearance of the lens itself (Flickinger et al., 1955; ten Cate and van 

 Doorenmaalen, 1950). Even in the head ectoderm wherefrom the lens is 

 formed, or in the eye vesicles, which induce the transformation of head 

 ectoderm into lens, no lens antigen is found by the precipitation method 

 (Woerdeman, 1955). With the Coons method for localizing antigen 

 on histological sections which uses a fluorescent antiserum as a specific 

 stain, a characterized localization was found only when the lens was formed ; 

 before this, some lens antiserum was bound everywhere. It may be there- 

 fore that typical lens proteins were made before, but in small amounts, by 

 all the cells of the embryo, and that their formation was later restricted to a 

 few territories. 



Ebert (1954, 1958, 1959) did a similar study on the appearance of heart 

 myosin in the chick embryo. At the end of gastrulation, a protein which 

 reacts with anticardiac myosin serum arises everywhere in the embryo; 

 later this protein is restricted to the heart-forming area. Actin, on the con- 

 trary, is not detected except in this area, and only when myosin localization 

 is already restricted to it. 



The appearance of skeletal muscle myosin and of collagen have also been 

 studied by similar and other methods ; and a wealth of data on the changes 

 in many enzymic activities during development can be found in the 

 literature of chemical embryology (see Brachet, 1944, 1960). 



In a way, all these data can be taken as more elements to be added to the 

 morphological observations ; they are an extension of morphology down to 

 the chemical level. As a rule, specific proteins appear indeed at the same 

 time as the corresponding organ. It may be argued that the question as to 

 which comes first, specific proteins or morphological differentiation, is not 

 very consistent: in so intricate a process as development, where each step 

 may influence the steps to come in very indirect ways, it is certainly not 

 easy to distinguish causes from effects. But proteins are the most immediate 

 expression of the genetic characters, and the enzymes do control all the 

 biochemical operations by which cell material is made. Proteins therefore 

 must occupy a key position in differentiation. As the mechanism of protein 

 synthesis and its control are becoming better and better understood, the 



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