152 



THE BIOSYNTHESIS OF PROTEINS 



Study of changes of protein synthesis during development is certainly a 

 promising approach to some of the basic problems of embryology. 



A fundamental question is whether two differentiated cells of the same 

 organism produce two different sets of proteins because they have a differ- 

 ent complement of mendelian genes. This is, in newer words and in a more 

 restrictive sense, the problem of the equipotentiality of the nuclei which 

 retained the attention of Driesch (1894) and of Morgan (1934). 



The amount of DNA in a diploid somatic cell is just double that of the 

 spermatozoon. The number of chromosomes and their general shape 



Z7A Myosin 



Actin + Myosin 



Fig. 36. Distribution of actin and myosin in the early chick embryo at 

 successive stages of development, as shown by Coon's method (Ebert, 



1954). 



usually do not change during development. In insects, certain tissues con- 

 tain enormously enlarged chromosomes, and a correlation has been estab- 

 lished between the visible bands of the chromosomes and the genetic map 

 computed from recombination frequencies. The same pattern of bands is 

 found in the chromosomes of the salivary gland and of the midgut of 

 Drosophila (Berger, 1940) or in the rectum, the Malpighian tubes, the 

 midgut and the salivary glands of Sciara (Beermann, 1952); there is no 

 evidence of any change in structure of these chromosomes during differ- 

 entiation. These are indications that not many genes are lost on the way; 

 but subtle changes in the state of the genes might escape these crude 



