In Pursuit of the Gene

154 ¨ CELL BIOLOGY
tween nuclear divisions, reappeared in roughly their old positions, and retained
their sizes, shapes, and number. “The chromosomes,” Boveri now asserted,
“are autonomous individuals that retain this autonomy even in the
resting nucleus.” 29 In the conclusion of the paper he took the argument one
step further, suggesting that each of the chromosomes of Ascaris might contain
“different qualities.”
The remarkable sea urchin, which had played the leading role in the
development of the Hertwig’s fertilization theory in 1875, took center stage
once again at the end of the century when Boveri saw how it could be used
to provide definitive experimental proof of the role of the nucleus and
chromosomes in heredity. As Oscar Hertwig had first noted, sea urchin eggs
were occasionally fertilized by two sperm simultaneously, and these doubly
fertilized eggs bypassed the normal two-cell stage of development and instead
went directly from one to four cells on the way to becoming fully
formed larvae. 30 However, these tetrafoils, or simultaneous fours as they were
more commonly called, never became healthy larvae, instead suffering
from a variety of developmental irregularities and, most often, premature
death.
Around 1901, Boveri saw how his early work on the mechanism of fertilization
and cell division in the sea urchin could be used to prove that
each chromosome in an organism was a unique individual containing different
qualities. Already in 1888 he had shown that in addition to injecting
its nucleus into the egg, the sperm donated a small circular body known as
the centrosome, and that, independently of the nucleus, the sperm centrosome
divided into two parts, which moved to opposite sides of the cell
where they acted as attractive centers. Furthermore, he observed that
threads emanated from each of these centrosomes and attached to opposite
sides of each of the recently divided chromosomes, which were then
dragged to opposite ends of the cell. While the newly fertilized egg containing
one divided centrosome was an exquisitely well-designed machine to
evenly distribute one set of longitudinally divided chromosomes to two
poles, Boveri realized that the doubly fertilized egg, which received two
sperm centrosomes that gave rise to four attractive centers, would not be
nearly so effective. When four attractive centers were competing to attach
to each split chromosome, Boveri conjectured, it would simply be a matter