In Pursuit of the Gene

152 ¨ CELL BIOLOGY
generation. This was the very same argument Galton had first put forward
in 1871, and Weismann would later acknowledge his priority. 20
There were, however, troubling reports issuing from Flemming and
others indicating that the second division was, like the standard mitotic division,
a separation of longitudinally split daughter chromosomes, and thus
could not result in a reduction of the nuclear material of the egg-cell by
half. Despite evidence to the contrary, the theoretical necessity for a halving
of the genetic material was so compelling to Weismann that he was
willing to disregard the empirical evidence: “Even if I am mistaken in this
interpretation, the theoretical necessity for a reduction ...seems to me to
be so securely founded that the process by which it is effected must take
place, even if they are not supplied by the facts already ascertained.” 21 Although
he would be reviled in some quarters for his failure to subordinate
theory to empirical evidence, he defended his theoretical approach. “To go
on investigating without the guidance of theories,” he replied to his critics,
“is like attempting to walk in a thick mist without a track and without a
compass.” 22
Weismann’s prediction that two distinct types of cell divisions would be
found to be involved in the maturation of the sex cells—in the first type of
division, which Weismann termed “equal division,” each daughter nuclei
inherited a full complement of hereditary material by the process of longitudinal
splitting of chromosomes, and in the second, a reductive division,
each daughter nucleus received only half the number of hereditary determinants—proved
to be prescient. In the words of the Princeton geneticist
Edwin G. Conklin, it was “as brilliant an example of scientific prophecy as
was the prediction of the existence of the planet Neptune.” 23 The second
process would later be termed “meiosis,” from the Greek for “that which
is reduced.” 24 Although Weismann got some of the details wrong—the
reductive division turned out to precede one final mitotic division in the
formation of the sex cells—he had grasped an elemental fact.
¨ HOWEVER FASCINATING the speculations brought forth about the
role of the nucleus and the behavior of the chromosomes in heredity,
they were not satisfactory to the young German biologist Theodor Boveri,