Monitoring Stem Cell Research 
79 
with that afforded to fully human subjects.®° They suggest that 
genetic identity and organismic continuity are not sufficient; 
what matters is present form and function, more than mere 
potential.®^ 
Several particular putative discontinuities (and combina- 
tions of them) have been proposed as candidates for the divi- 
sion between early stages, when a human embryo may be dis- 
aggregated for research, and later stages, when it deserves 
some greater level of protection. 
(a) Primitive streak: The most popular candidate for a mean- 
ingful point of discontinuity is the appearance of the primi- 
tive streak, the earliest visible “structure” that defines the 
region of the embryo along which the vertebral column will 
form. The primitive streak generally appears around the 14^^ 
day after first cell division. It is taken to indicate the ante- 
rior-posterior axis of an embryo (in vertebrates), although 
recent studies suggest that polarity may be established 
much earlier, and in fact may be indicated by the point at 
which the sperm enters the egg.®^ 
A principal reason for the importance placed on the primi- 
tive streak has to do with the biology of twinning. Prior to 
the appearance of the primitive streak, an embryo may 
(rarely, and for imknown reasons) divide completely to form 
identical twins. Some conclude that individuality must not 
yet be established, since the embryo might yet become two 
embryos.®® Since individuality is essential to human person- 
hood and capacity for moral status, since individuality pre- 
sumes a definitive single individual, and since the singular- 
ity of the embryo is not irrevocably settled prior to the ap- 
pearance of the primitive streak, they argue that the entity 
prior to the primitive streak stage lacks definitive individu- 
ality and hence also moral status.®"^ Critics of this line of rea- 
soning point to the low statistical probability of monozy- 
gotic twinning: one in 240 live births (though rates clearly 
vary among populations in different regions, and precise 
figures probably cannot be known because some zygotes 
that split likely later fuse into one, and some singleton 
births may conceal a twin who died early in gestation). Crit- 
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