JOSHUA LEDERBERG 
protein synthesis which underlies the orderly differentiation 
of cell types—how some DNA segments are made to call out 
their instructions and others are suppressed. These issues are 
now suddenly accessible to experimental analysis. Embryology 
is very much in the situation of atomic physics in 1900; having 
had an honourable and successful tradition it is about to begin! 
But it will not take long to mature. Most predictions of research 
progress have proved recently to be far too conservative. 
Until now, the major problems of human development—not 
only embryology, but also the phenomena of learning (in its 
neurobiological aspects), immunity (with its bearing on trans- 
plantation), neoplasia and senescence—could be approached 
at only the most superficial level. They are about to be trans- 
formed in the sense that genetics has been, as epiphenomena of 
protein and nucleic acid synthesis. The present intensity of 
effort suggests a span of from five to no more than twenty years 
for an analogous systematization. The application of these 
advances to human affairs is equally imminent. 
On these premises it would be incredible if we did not soon 
have the basis of developmental engineering technique to regu- 
late, for example, the size of the human brain by prenatal or 
early postnatal intervention. In fact, it is astonishing how little 
experimental work has been done to test some elementary 
questions on the hormonal regulation of brain size in laboratory 
animals or the functional interconnexion of supernumerary 
brains. Needless to say, “brain size”’ and “‘intelligence”’ should 
be read as euphemisms for whatever each of us projects as the 
ideal of human personality. 
The basic concept of molecular biology is the chain of infor- 
mation from DNA to ribonucleic acid (RNA) to protein. We 
are just beginning to ask questions about mental mechanisms 
from this standpoint. The simplest and one of the oldest 
suggestions about memory is the modification of neuronal inter- 
connexion through control of synthesis and deposition of durable 
proteins at the interfaces. The link between electrical impulses 
and protein synthesis could easily be the accompanying shifts 
of potassium and sodium ion concentrations, these ions being 
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