Control of Reproduction in Mammals 
by administration of the antiprogestin later rather than earlier 
in the pre-implantation period. To our surprise, administration 
on day 1 alone was uniformly effective whereas administration 
on day 3 alone was either ineffective or very much less effective. 
Indeed, these same antiprogestins administered on day 8 or 
later were practically ineffective in causing death or resorption 
of the embryo. We have, therefore, a group of compounds which 
will uniformly prevent the free ovum from becoming implanted 
provided that they are administered on the day following mating. 
This unexpected effect of compounds originally chosen 
because of antiprogestational activity on the endometrium raises 
many questions concerning the factors and processes involved 
in free ovum development. Thus normal-appearing blastocysts 
are recovered from the uteri of animals receiving sterilizing 
doses of antiprogestins. Why do they fail to implant? A large 
proportion of sterile women apparently ovulate normally but 
fail to have implantations. Is it possible that certain steroids 
produced by the ovaries or adrenal glands of these women act 
the way our implantation-inhibiting steroids do? How will 
these compounds affect human reproductive processes? It is, 
I believe, clear that many interesting problems remain for study. 
CONCLUSIONS 
I have attempted to present here the highlights and to dwell 
briefly on some implications of investigations into aspects of the 
life history of mammalian eggs. To me it is a most fascinating 
field for research, perhaps because we are indeed just at the 
forefront of an era where these and numerous other mysteries 
of reproduction will no longer be mysteries. But it is clear even 
now that exceptionally effective methods of fertility control 
are at hand and that others are in the offing. The most promis- 
ing are those concerned with inhibition of the development of 
the ovum, and this is perhaps auspicious since it is women who 
must bear the major burdens of reproduction and who, indeed, 
have been the major seekers of relief from these burdens. 
In presenting the history of certain special efforts in fertility 
control, I have attempted to indicate how facts derived from 
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