World Population 
Pincus: Vasectomy has been reversed experimentally by my 
colleagues, Dr. John Rock and Dr. Celso Garcia. In animals it 
is easy, if you do the operation properly. In man they have 
found that it is possible sometimes to undertake repair, but if 
there is much inflammation at the site of the original operation 
so much of the tubular apparatus will be destroyed that, at 
least by their method, it is impossible to restore it. On the 
other hand, if there is an absolute minimum of inflammation and 
essentially just the closure of an aperture, its reopening by a 
very simple means is not difficult. This is a technical surgical 
matter which in the future I am sure will be solved. 
Hoagland: Dr. Rock and Dr. Garcia report 40 per cent now 
reversible. 
Lederberg: We should think also of the possibility of muta- 
tions being produced either by the effect of hot baths on 
spermatogonia or by the cold storage of sperm for artificial 
insemination. Suppose we set as a ‘“‘standard of evil” ten 
times the most pessimistic estimates of the mutagenic effects 
of fallout; we cannot exclude the possibility that hot baths or 
cold storage of sperm might have an equally deleterious 
effect, or even a beneficial one, and it would be a long time 
before we could collect enough data to be reassuring on that 
score. 
Hoagland: What is the status of “rhythm regulation” at 
present ? 
Pincus: I don’t know of any studies which do any more than 
we have done, namely to get a succession of normal menstrual 
cycles but—unfortunately, from the point of view of the rhythm 
method—complicated by lack of ovulation. However, there 
are some new steroids which appear not to inhibit ovulation 
very markedly and which do regulate menstrual cyclicity. So 
far no accurate studies have been made on these compounds, 
but they may turn out to provide the answer. 
Szent-Gyérgyi: I should like to mention an accidental 
observation we have made recently. We were interested in 
whether the thymus gland has a hormonal function. The 
thymus gland is most strongly developed in young animals and 
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