80 
Ethical and Policy Developments 
ics also point to evidence that twinning results, not from an 
intrinsic drive within the embryo, but from a disruption of 
the fragile cell dynamics of embryogenesis. Evidence for 
this, they suggest, may be seen in the increased incidence 
of monozygotic twinning (up to ten fold in blastocyst trans- 
fer) associated with IVF. This suggests, in their view, that 
twinning is neither a proof of the absence of an integrated 
individual organism with a drive in the direction of devel- 
opment nor a demonstration ex post facto of the absence of 
moral worth of the embryo before twinning.®^ 
Nonetheless, for this reason, and for others (discussed be- 
low) having to do with the formation of the nervous system, 
the primitive streak has often been taken to be a highly sig- 
nificant marker of embryological development, and many 
commentators suggest it as a reasonable candidate for a 
meaningful point of discontinuity. For this reason, many 
supporters of embryo research regularly propose the 14^^ 
day of development as a logical stopping point for permis- 
sible embryo research.®® 
(b) Nervous system: A second argument for discontinuity fo- 
cuses on the developing nervous system. Many observers 
regard the nervous system as an especially important 
marker of humanity, both because the human brain is criti- 
cal for all “higher” human activities, and because the nerv- 
ous system is the seat of sensation and, especially relevant 
to this case, the sensation of pain. Proponents of this view 
hold that before an embryo has developed the capacity for 
feeling pain (or, in some forms of the argument, before sen- 
tience), we cross no crucial moral boundary in subjecting it 
to destructive research.®*^ For some, this is taken to mean 
that the primitive streak, as the first marker of a future 
nervous system, is a crucial feature of developing life. For 
others, only later points of neural development (where pain 
might plausibly be experienced) are held to be decisive.®® 
Critics meanwhile contend that neural development as well 
as development of other systems (such as the cardio- 
vascular system) are the natural outcome of the genetic 
program in action, and should be explained by reference to 
PRE -PUBLICATION VERSION 
