Biological Future of Man 
also important cofactors for several enzymes involved in protein 
synthesis. More elaborate coding, such as the modulations of 
the actual conformation of the proteins can also be invoked, but 
may not be necessary to account for the actual storage capacity 
of the brain. Speculative models for this kind of coding can be 
built on the basis of present knowledge of protein synthesis, 
without impairing the conservation of information in the nucleic 
acids or invoking unsubstantiated principles of electrical control 
of nucleic sequences. Unlike other cellular systems, the neu- 
rones, which rarely if ever divide, need no mechanism to 
propagate their information to cell progeny. The burden of 
data storage may therefore be confided entirely to protein. 
The purpose of mentioning these speculations is to dramatize 
the relationship of mental science to molecular biology. The 
analysis of protein structure and metabolism throughout the 
brain, the correlation of structural development with learning, 
its genotypic control, and its alteration in disease are beginning 
to be attacked in force, impelled in part by social concern for 
the immensely important problem of mental retardation, as 
such research must tell us even more about normal mental 
development. 
In another field of developmental engineering Professor Meda- 
war has already exhibited a tour de force, the abolition of 
immunity to transplants introduced in early life, a work which 
has clarified the biology of immunity and points to the solution 
of the transplantation problem. At present human individuality 
is the obstacle to spare-part medicine: the organism rejects 
grafts from other individuals, even though the alien tissue might 
be a life-extending kidney or heart. Why the chemistry of our 
cell membranes should be so individualized is not clear; it may 
impede the contagious spread of cancer cells, or perhaps of 
viruses which attack host cell surfaces. 
There is little evidence of forethought about the social impact 
of the solution to the homograft problem, although this solution 
seems very near and may prove a prototype for the exercise of 
responsible power in biological engineering. Nor has the full 
impact of tissue replacement on the practice of medicine been 
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