68 Therapeutic Cloning question is closely bound up with the principle of division of labor the first multicellular organism was probably a cluster of similar cells, but these units soon lost their original homo- geneity: the single cluster would come to be divided into two groups of cells, which may be called somatic and reproductive. As the complexity of the metazoan body increased, these two groups became more sharply separated from each other. Very soon the somatic cells surpassed the reproductive in number, and during this increase they became more and more broken up by the division of labor into sharply separated systems of tissues. As these changes took place, the power of reproduc- ing large parts of the organism was lost, while the power of reproducing the whole individual became concentrated in the reproductive cells alone. But, it does not therefore follow that the somatic cells were compelled to lose the power of unlim- ited cell reproduction. So, Weismann made the astonishing prediction that while the germ-line cells of multicellular animals, such as humans, were  immortal  (specifically,  they  could  replicate  without limit), the somatic cells were in fact mortal – that is, they had the capacity to divide only a finite number of divisions: Death takes place because a worn-out tissue cannot for- ever renew itself, and because a capacity for increase by means of cell-division is not everlasting, but finite. [2] HAYFLICK’S EXPERIMENT In 1961, the cell biologist Leonard Hayflick published the seminal work that convinced the scientific community that cells in the human body – the somatic cells – are mortal. [3] They  could  divide  and  proliferate,  but  as  Weismann  had predicted so many years earlier, even with optimum growth