76 
Ethical and Policy Developments 
that some particular point of discontinuity (or the sum of sev- 
eral such points) marks a morally significant distinction be- 
tween stages, which difference should guide our treatment of 
human embryos. 
1. The Case for Continuity. 
Many of those who seek to defend human embryos base 
their case on some form of the argument for biological continu- 
ity and sameness through time. For example, they argue that a 
human embryo is an organic whole, a living member of the 
human species in the earliest stage of natural development, 
and that, given the appropriate environment, it will, by self- 
directed integral organic functioning, develop progressively to 
the next more mature stage and become first a human fetus 
and then a human infant. Every adult human being around us, 
they argue, is the same individual who, at an earlier stage of 
life, was a human embryo. We all were then, as we still are 
now, distinct and complete human organisms, not mere parts 
of other organisms. 
This view holds that only the very beginning of a new (em- 
bryonic) life can serve as a reasonable boundary line in accord- 
ing moral worth to a human organism, because it is the mo- 
ment marked out by nature for the first visible appearance in 
the world of a new individual. Before fertilization, no new indi- 
vidual exists. After it, sperm and egg cells are gone — 
subsumed and transformed into a new, third entity capable of 
its own internally self-directed development. By itself, no 
sperm or egg has the potential to become an adult, but zy- 
gotes by their very nature do.^® 
Many authors therefore regard the activation of the oocyte 
(by the penetration of the sperm)^® or the completion of syn- 
gamy (the combining of paternally- and maternally-contributed 
haploid pro-nuclei to result in a unique diploid nucleus of a de- 
veloping zygote) as a meaningful marker of the beginning of a 
new human life worthy of protection.®® After this point, there is 
a new genome, in a new individual organism, and there is a 
zygote (single-celled embryo) already beginning its first cleav- 
age and embarking on its continuous developmental path to- 
ward birth. 
PRE -PUBLICATION VERSION 
