One reason why a sheep, a less well-understood experimental subject than the laboratory mo

One reason why a sheep, a less well-understood experimental subject than

the laboratory mouse, should have proved easier to clone may stem from

differences in the initial stages of the two species' embryonic development.

After reaching maturity in the ovary of the mother, the unfertilized eggs of all

(5) mammals accumulate a supply of proteins, and the means of producing fresh

protein. In this way, the mammalian egg brings with it a larder for the embryo

to make use of until its own genes activate and supply this requirement

themselves. The sheep embryo disposes of its store properly and need not

depend on its own genes until the sixteen-cell stage, four cell divisions

(10) successive to fertilization, while in contrast, the mouse embryo commences this

process more precociously, becoming reliant on the activity of its own genes

after just the first division when the fertilized egg becomes two cells.

Therefore, a foreign nucleus introduced into a sheep egg exploits a respite in its

host's biological development, allowing it to adapt to its new role before

(15) assuming genetic control.

Concomitantly, a nucleus introduced into a mouse egg must acclimatize

quickly for its genes to be able to direct embryonic development within a single

cell division, so perhaps there is insufficient time for the extensive re-

programming of compulsory gene activity. The human embryo is thought to rely

(20) on its own genes after three cellular divisions, which might or might not

provide time enough for a foreign nucleus to acclimate. However, were

scientists to comprehend the nature of the indispensable re-programming then

there is every likelihood that both mice and humans could be cloned.

Despite the long-standing availability of this technology, there has until

(25) recently been little interest in it. Some people suffering from infertility as a

result of rare hereditary diseases could produce offspring, but cloned individuals

may be at risk given scientists' limited knowledge of the long term effects of

allowing an "old" adult cell nucleus to commence life again in an egg. The

nucleus of a skin cell could have accumulated a multitude of genetic mistakes of

(30) no consequence to its role in the skin, but the same cell could prove deleterious

in other tissues, or immensely increase the probability of the affliction with

cancer. The threat to general human health posed by cloning, as opposed to the

individual, is difficult to determine, but the risks are almost certainly lower

than those encountered in the effective inbreeding of consanguine marriages,

(35) and thus there are no scientific grounds per se for banning cloning. Like other

practices inconsequential to the physical well being of humanity, but generally

deemed undesirable on moral or social grounds, the prohibition of human

cloning will ultimately rest with only a simple pragmatic decision.

In this passage, the author is primarily concerned with

A．advancing an argument against human cloning based on evidence from cloning experiments performed on lower life forms

B．measuring the technical and ethical limitations of the embryonic cloning process in mammals

C．refuting arguments against human cloning through the use of hard scientific evidence

D．illuminating the critical ethical distinctions in process between human cloning and sheep cloning

E．describing how the ethical issues in the quest for human cloning have given rise to new models of embryonic development