In Pursuit of the Gene

74 ¨ PANGENES
his own. In the place of Darwin’s vast numbers of gemmules, Weismann
proposed the existence of “germ-plasm,” a single indivisible unit that was
thought to be capable of regenerating a complete individual. Unlike Darwin’s
changeable gemmules, the germ-plasm, located in specialized germ
cells, was passed down through the generations, “uninfluenced by that
which happens during the life of the individual which bears it.” 10 In Weismann’s
developmental model, it was only the germ-plasm in the germ cells
that remained inviolate, while the germ-plasm in the other cells of the
body underwent a winnowing process, shedding hereditary material during
successive divisions and retaining only the portion of the germ-plasm necessary
to perform each cell’s specialized function.
As De Vries hastened to point out, Weismann’s theory flew in the face
of one of the most prosaic of botanical facts, namely, that a whole plant
could be regenerated from any of a variety of specialized cell types. To get
around this obvious objection, De Vries proposed that the nuclei of all cells
contained an identical collection of pangenes that were the repository of
the totality of the hereditary information. The distinctive nature of different
cell types was not a matter of the hereditary endowment of the cell,
but was instead determined by the types and numbers of the particular
pangenes that were transported from the nucleus to the cytoplasm. As he
explained it in a popular magazine of the day, “We can say: the instructions
lie in the nucleus, the realization takes place in the protoplasm.” 11 This idea
would turn out to be the crux of the modern understanding of the process
of cell differentiation.
Intracellular Pangenesis, published in 1889, was the culmination of more
than a decade of experimental and theoretical work that began in the late
1870s when De Vries had combed the countryside near Amsterdam looking
for strange irregularities in plant shapes and forms—so-called monstrosities—that
might illuminate the nature of the pangenes. His goal, as he
later explained, was to create a “herbarium of monstrosities.” Following the
tried-and-true method of professional plant breeders, who were always on
the lookout for promising new shapes and colors from which to create new
varieties, De Vries self-fertilized his most promising specimens and selected